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Full text of "Handbook of physiology; a critical, comprehensive presentation of physiological knowledge and concepts"

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HANDBOOK OF PHYSIOLOGY 



SECTION 1: Neurophysiology, volume i 



HANDBOOK OF PHYSIOLOGY 

A critical, comprehensive presentation 
of physiological knowledge and concepts 



SECTION 1: 



Neurophysiology 



VOLUME I 



Editor-in-Chief: JOHN FIELD 
Section Editor: H. W. MAGOUN 
Executive Editor: VICTOR E. HALL 



American Physiological Society, Washington, d. c, 1959 



@ Copyrighl ig5<), American Physiological Society 

Library of Congress Catalog Card No. ^g-isg^y 

Printed m the United States of America by Waverly Press, Inc., Baltimore 2, .Maryland 

Distributed by Williams & M'ilkins Co., Baltimore 2, Maryland 



Foreword 



The original literature in the field of physiology 
has become so vast and is growing so rapidlv that 
the retrieval, correlation and evaluation of knowledge 
has become with each passing year a more complex 
and pressing problem. Compounding the difficulties 
has been the inevitable trend toward fragmentation 
into smaller and smaller compartments, both of 
knowledge and of research skills. This trend is not 
only inevitable, but it is necessary to healthy growth. 
It must, however, be accompanied by the develop- 
ment of mechanisms for convenient and reliable re- 
integration in order that knowledge shall not be lost 
and research efTort wasted. 



The American Physiological Society has enlisted 
the cooperation of physiological scientists over the 
world in attempting to provide a mechanism in this 
Handbook of Physiology series for providing a com- 
prehensive but critical presentation of the state 
of knowledge in the various fields of functional 
biology. It is intended to cover the physiological 
sciences in their entirety once in about ten years, 
and to repeat the process periodically thereafter. 

Board of Publication Trustees 
MAURICE B. visscHER, Chairman 

W I L L I .^ M F. H .^ M I L T O N 
PHILIP BARD 



Preface 



This Handbook of Physiology , like its predecessors from 
von Haller on, is designed to constitute a repository 
for the body of present pliysioiogical knowledge, 
systematically organized and presented. It is addressed 
primarily to professional physiologists and advanced 
students in physiology and related fields. Its purpose 
is to enable such readers, by perusal of any Section, 
to obtain a working grasp of the concepts of that field 
and of their experimental background sufficient for 
initial planning of research projects or preparation 
for teaching. 

To accomplish this purpose the editors have 
planned a book which would differ from textbooks in 
being more complete, more analytical and more 
authoritative. It would differ from a series of mono- 
graphs in being organized on a consistent plan with- 
out important gaps between topics and with as nearly 
as possible the same relation of intensity of coverage 
to importance of topic throughout. It would differ 
from publications emphasizing new developments in 
that the background of currently accepted or classical 
concepts would be set forth, newer ideas receiving 
not more than their due proportion of emphasis 
relative to the whole body of knowledge in the field. 
Finally it would differ from a collection of original 
papers on a series of topics in that it would provide an 
integrated condensation and evaluation of the mate- 
rial contained therein. Moreover, the overall plan 
provides that the key experimental findings in the 
development of each field of investigation be de- 
scribed and discussed in sufficient detail (with appro- 
priate illustrations, quantitati\e data and adequate 
documentation) to make clear their nature, validity 
and significance for the fundamental concepts of the 
field. The success of this endeavor must be left to the 
reader's judgment. 



This Handbook stands as the current representative 
of an historic series of efforts to collect and system- 
atize biological knowledge — a series continued when 
the Board of Publication Trustees of the American 
Physiological Society decided in 195;] to sponsor the 
present undertaking. A brief list of notable prede- 
cessors may interest .some readers. First known of 
the series is a brief Sumerian 'pharmacopeia" dating 
from perhaps 2100 B.C. Later examples included 
several Egyptian papyri such as the Ebers and the 
Edwin Smith. Far more extensive compilations char- 
acterized the Greco-Roman period. Outstanding 
among those were the Hippocratic collection (written 
ijy several authors) and the encyclopedic writings 
associated with the names of Aristotle, Theophrastus, 
Celsus and Galen (Pliny's work is useful chieflv to 
the student of folklore). These treatises systematized 
knowledge of the day over a wide range and set forth 
new information based on the authors' observations. 
Thus they combined the roles of handbook and scien- 
tific journal, a pattern that persisted until develop- 
ment of scientific journals (in the seventeenth cen- 
tury). Other important compilations were made by 
the writers of the 'Moslem Renaissance' such as Rhazes 
and Avicenna, to whom much of the Greco-Roman 
literature was available. 

European biological compendia of the Christian 
era, from the fourth century Physiologus to the exten- 
sive biological encyclopedias of the sixteenth and 
seventeenth centuries, differed greatly in character 
from Greco-Roman and 'Moslem Renaissance' work. 
Marked by strong theological and anthropocentric 
orientation, they lacked the descriptive accuracy and 
rational approach of the ancients. Scientia was con- 
sidered ancillary to sapientia. Nature was studied 
chiefly to obtain illustrations for moral tales and 



HANDBOOK OF PHVSIOI.OGV 



NEUROPHYSIOLOGY I 



religious dogmas, not to gain knowledge or insight, 
or to learn how to manipulate and control the en- 
vironment. Writers showed little critical capacity and 
failed to distinguish between the tfue and the fabu- 
lous, the important and the trivial. These elements 
are .still evident in such major sixteenth century 
biological encyclopedias as Gesner's Historiae Ant- 
malium (5 volumes, 1551 1587), and Aldrovandi's 
Opera Omnia (13 volumes, 1399-1677). In both the 
mark of the medieval Bestiary is strong. 

However, the tide was turning in the sixteenth 
century despite these notable examples of medieval 
Weltanschauung. The range and precision of anatomi- 
cal knowledge were greatly extended by publication 
in 1543 of Vesalius' De Hiimani Corporis Fabrica. It is 
interesting to note that increasingly accurate hand- 
books of descriptive botany began to appear. At about 
this time the great transition from the medieval to 
the modern outlook (the 'scientific revolution of 1500- 
1800') was under way. This has been succinctly 
described by Raven: "Little by little, nonsense was 
recognized, fables were exploded, superstitions were 
unmasked and the world outlook built up out of 
these elements fell to pieces. The seemingly irrelevant 
labors of men like Turner or Penny to identify and 
name and describe bore fruit in a refusal to accept 
tradition on authority and in an insistence that state- 
ments must be based upon observation and capable 
of verification" (C. E. Raven. English Naturalists 
from Neckam to Ray. 1947, p. 227). 

The rise of the mechanical philosophy in the seven- 
teenth century and the rationalism of the eighteenth 
furnished an intellectual climate favorable for science. 
This was reflected in the papers, monographs and 
compendia produced. In the spirit of the time, 
Diderot, d'Alembert and their associates prepared 
the Encyclopedie ou Dictionnaire Raisonne Des Sciences, 
Des Arts et Des Metiers (35 volumes, Paris, i 751-1 780). 
While the major contribution of this influential work 
was to diffuse the rationalist interpretation of the 
universe in mechanistic terms, it included many con- 
tributions in the biological sciences. Together these 
constitute a transitional stage of biological handbook 
— quite modern in spirit but not in respect of fact or 
concept. 

While the Encyclopedic was in preparation in Paris, 
the Swiss savant Albrecht von Haller was compiling 
the Elementa Physiologiae Corporis Humani (8 volumes, 
Lausanne, 1 757-1 765). This comprised both a hand- 
book of anatomy and physiology and a vehicle for 
publication of much original work by the author. 
Compared to earlier work the writing shows impres- 



sive critical capacity, detailed familiarity with tlie 
achievements of others, ability to distinguish the trivial 
and the important and over-all scientific insight. This 
was the first of the great series of German Handhuch 
of physiology. 

The vast increase in scientific activity, with multi- 
plication of investigators, laboratories and journals, 
that characterized the nineteenth century led to more 
frequent collection and systematization of knowledge 
in the several active fields. This was naturally centered 
in Germany where scientific activity was greatest. 
Notable examples of handbooks of physiology were 
R. Wagner's Handworterhuch der Physiologic mit Ruch- 
sicht aiif Physiologisches Pathologic (Braunschweig, 
1 842-1 853); L. Hermann's Handhuch der Physiulogie 
(Leipzig, 1 879- 1 883); G. Richet's unfinished Dic- 
tionnaire de Physiologic (Paris, 1 895-1 928); E. A. 
vSchafer's Text-Book of Physiology (Edinburgh and 
London, 1898- 1900); W. Nagel's Handhuch der 
Physiologic des Menschen (Leipzig, 1905-1910); the 
massive Handhuch der Normalen und Pathologischen 
Physiologic, mit Berikksichtigung der Experimentellcn 
Pharmakologie, edited by A. Bethe, G. von Bergmann, 
G. Embden and A. Ellinger (Berlin, 1 926-1 932); 
and our immediate predecessor, G.-H. Roger and 
L. Billet's Traite de Physiologic Normale et Pathologique 
(Paris, 1 933-1 940). Characteristically these hand- 
books comprised the contributions of many authors 
and, in the last two, collaboration of several editors 
as well. These, with comparable coitipilations in 
cognate fields such as K. von Bardeleben's Handhuch 
der Anatomic des Menschen (Jena, 1896-1911) and 
E. Abderhalden's Handhuch der Biologischen Arheits- 
methoden (Berlin, 1 925-1 939), have provided a corpus 
of collected and systematized scientific knowledge. A 
notable feature of all handbooks, including the pres- 
ent one, is their increasingly international character, 
reflecting the broadening base of the world of science. 

Survey of these codifications from the earliest on 
provides a basis for Abraham Flexner's trenchant 
comment on the history of medicine. "From the 
earliest times medicine has been a curious blend of 
superstition, empiricism, and that kind of sagacious 
observation which is the stuff out of which ultimately 
science is made. Of these three strands — superstition, 
empiricism and observation — medicine was consti- 
tuted in the days of the priest-physicians of Egypt and 
Babylonia; of the same three strands it is still com- 
posed. The proportions have, however, varied sig- 
nificantly; an increasingly alert and determined 
effort, running through the ages, has endeavored to 
expell superstition, to narrow the range of empiricism 



PREFACE 



and to enlarge, refine and systematize the scope of 
observation. . . . The general trend of medicine has 
been away from magic and empiricism and in the 
direction of rationality and definiteness" (A. Flexner. 
Medical Education. A Comparative Study. New York, 
1925). We trust that continuation of this trend is 
reflected in this Handbook. 

It is difficult to acknowledge properly the devoted 
and effective work which has made this vast under- 



taking possible. Its success is due alike to the con- 
tributors, to the editorial staff and to the Board of 
Publication Trustees of the American Physiological 
Society. Alike to all of these is due the gratitude of the 
world of physiologists for a task well done. 

JOHN FIELD 

Editor-in-Chief, ig§4-ig§8 



Preface to the Section on Neurophysiology 



As the Editor-in-Chief has pointed out, tlie decision 
of the American Physiological Society to sponsor a 
Handbook of Physiology continues an historic series of 
efforts to collect and systematize knowledge in more 
readily available forms. Although sharing many of 
the features of its predecessors, the present Handbook 
of Physiology is likely to be less formidable than most 
of them. Its goal, like that of chariot racing, has been 
to secure a balanced perch astride the rushing progress 
of investigative advance. It attempts to survey the 
status of physiology just past the mid-mark of the 
twentieth century. In the case of each topic, the com- 
pilative accumulation of analytic data is either intro- 
duced or concluded by synthesizing comments of an 
'elder statesman' still active in the field. Thus a bal- 
ance is sought between the presentation of specific 
information and conceptualization appropriate to it. 

Appropriately also, the Handbook begins with con- 
sideration of the nervous system by which the activities 
of other portions of the body are coordinated and 
controlled. The nervous system remains the last organ 
of the body still formidably to resist investigative 
attack; many fundamental concepts of its function lie 
waiting in the future. Views proposing a spiritual 
basis for neural function have obtained since classical 
antiquity. Only in the past century have materialistic 
outlooks been effectively introduced, first with respect 
to the nerve impulse, then in refle.x function and, most 
recently, in Russian views applying concepts of reflex 
physiolos^y to an understanding of higher activities 
of the brain. In this latter area, however, subjective 
experience and the mind still receive major attention 



in the West from the disciplines of psychology and 
psychiatry, a testimony to continuing dualistic points 
of view regarding function of the neural organ. In 
contemporary studies of physiological psychology the 
gap between brain and mind seems most rapidly to 
be closing; prominent representation of this field is 
probably the most novel feature of the table of con- 
tents of the present Neurophysiology Section. 

More than customarily, appreciation should be 
expressed to the contributing authors of this Hand- 
hook. Each has been willing to add to the many energy- 
draining burdens of a busy career the difficult task 
of surveying a field of investigative specialty both for 
the benefit of associates and for the general welfare 
of physiological science. The remarkably fine series 
of articles testifies to the generosity and skill of each 
contributor. It is to be hoped that reader appreciation 
may compensate these authors. 

Special gratitude should be expressed also for the 
efforts of the Executive Editor, Victor Hall. His back- 
ground of editorial experience with the Annual Review 
of Physiology enabled the manifold labors of this 
'sweet-blooded' man to be performed so deftly as 
perhaps to escape the attention of the general reader. 

Hopefully, all who use this Handbook will wish as 
I do to thank, if only silently, the contributing authors 
and the Executive Editor for their generous efforts 
and to applaud them for such a fine accomplishment. 



H. w. M A G o u N 
Section Editor 



Contents 



VII. 



VIII. 



XIII. 



XV. 



The historical development of 
neurophysiology 

MARY A. B. BRAZIER I 

Neuron physiology — Introduction 

J. C. ECCLES 59 

Conduction of the nerve impulse 

ICHIJI TASAKI 75 

Initiation of impulses at receptors 

J. A. B. GRAY 123 

Synaptic and ephaptic transmission 

HARRY GRUNDFEST 147 

Skeletal neuromuscular transmission 

PAUL FATT 199 

Autonomic neuroeffector transmission 

U. S. VON EULER 2 15 

Neuromuscular transmission in 
invertebrates 

E. J. FURSHPAN 239 

Brain potentials and rhythms — Introduction 

A. FESSARD 255 

Identification and analysis of single unit 
activity in the central nervous system 

KARL FRANK 261 

Intrinsic rhythms of the brain 

VV. GREY WALTER 279 

The evoked potentials 

HSIANG-TUNG CHANG 299 

Changes associated with forebrain 
excitation processes: d.c. potentials 
of the cerebral cortex 

JAMES L. o'lEARY 

SIDNEY GOLDRING 315 

The physiopathology of epileptic seizures 

HENRI GASTAUT 

M. FISCHER-WILLIAMS 329 

Sensory mechanisms — Introduction 

lord E. D. ADRIAN 365 



XIX. 



XXI. 



XVI. Nonphotic receptors in lower forms 

hansjochem autrum 369 

XVII. Touch and kinesthesis 
jerzv e. rose 

VERNON B. MOUNTCASTLE 387 

XVIII. Thermal sensations 

YNGVE ZOTTERM.'^N 43 1 

Pain 

WILLI.\M H. SWEET 459 

The sense of taste 

CARL PFAFFMANN 5O7 

The sense of smell 

W. R. ADEY 535 

Vestibular mechanisms 

B. E. GERNANDT 549 

Excitation of auditory receptors 

HALLOWELL DAVIS 565 

Central auditory mechanisms 

HARLOW W. ADES 585 

Vision — Introduction 

H. K. HARTLINE 615 

Photosensitivity in invertebrates 

LORUS J. MILNE 

MARGERY MILNE 62 I 

The image-forming mechanism of the eye 

GLENN A. FRY 647 

The photoreceptor process in vision 

GEORGE WALD 67 1 

Neural activity in the retina 

RAGN.-^R GRANIT 693 

Central mechanisms of vision 

S. HOWARD HARTLEY 713 

XXXI. Central control of receptors and sensory 
transmission systems 

ROBERT B. LIVINGSTON 74 1 

Index 761 



XXV. 



XXVI. 



XXX. 



CHAPTER I 



The historical development of neurophysiology 



MARY A. B. BRAZIER \ Ma'^sachusetts General Hospital, Boston, Massachusetts 



CHAPTER CONTENTS 

Early Concepts of Nervous Activity 

ENcitability anci Transmission in Nerves 

Spinal Cord and Reflex Activity 

Physiology of the Brain: Development of Ideas and Growth of 

Experiment 
Short List of Secondary Sources 
Biographies 



EARLY CONCEPTS OF NERVOUS ACTIVITY 

IN CONTRAST TO MEDICINE, a sciencc demanding 
synthesis of observations, experimental physiology, 
with its reliance on analysis and laboratory work, has 
little significant history before 1600. Leaders in 
medicine developed and practiced its therapies for 
many centuries before they felt the need to under- 
stand the nature and functions of the body's parts in 
any truly physiological sense and, when the urge for 
this knowledge first arose, it was to come as mucli 
from the philosophers as from the healers of the sick. 
Neurophysiology (a term not to come into use 
until centuries later) had as a legacy from the ancients 
only their speculative inferences and their primitive 
neuroanatomy. Aristotle had confounded nerves 
with tendons and ligaments, had thought the brain 
bloodless and the heart supreme, not only as a source 
of the nerves but as the seat of the soul. Herophilos 
and Erisistratos had recognized the brain as the 
center of the nervous svstem and the nerves as con- 
cerned both with sen.sation and movement. However, 
preliminary to all disciplines was the development of 
the scientific method and in this Aristotle was a fore- 
runner. If Aristotle is to be evaluated as a scientist, it 
must be admitted that he was almost always wrong in 



every inference he made from his \ast collections of 
natural history and numerous dis.sections; yet in spite 
of the stultifying effect of the ininujdcrate worship 
gi\en him by generations to follow, he stands out as a 
pioneer in the background of every scientific dis- 
cipline. He owes this position to his in\ention of a 
formal logic, and although his system lacked what the 
modern scientist uses most, namely hypothesis and 
induction, his was a first step towards the introduc- 
tion of logic as a tool for the scientist. Unfortunately 
Aristotle did not use his logic for this purpose him- 
.self ' As Francis Bacon put it, Aristotle "did not con- 
sult experience in order to make right propositions 
and axioms, but when he had settled his system to 
his will, he twisted experience round, and made her 
bend to his system." 

In the .second century A.D., Galen's experimental 
work added little to establish the functions of the 
animal structures he dissected, though the hypotheses 
he suggested were put forward so authoritatively 
that they remained unchallenged for nearly 1500 
years. To the intervening centuries, dominated as 
they were by the Christian church, the teleology 
implicit in Galen's approach was attractive. Early 
Western acquaintance with his writings depended 
entirely upon Latin translations of Arabic. It was only 
after the fall of the Byzantine Empire and the expul- 
sion of the Greek monks from the area of Turkish 
concjuest that the Greek language began to be read at 

' The fragments of Aristotle's writings that e.xist (probably 
his lecture notes} were not collected until more than lioo 
years after his death. His Opera were among the early scientific 
works to be printed (in Latin, 1472), nearly 1800 years after his 
death. English translations (The Works of Aristotle) were pub- 
lished by the Clarendon Press, Oxford, in several volumes 
between 1909 and 1931, edited by J. A. Smith and W. A. Ross. 



HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



all generally by scholars in Western Europe (i, 2). 
In the sixteenth century Thomas Linacre (3), physi- 
cian to Henry V'lII, who had taught Greek to Eras- 
mus at Oxford, translated some of Galen's works into 
Latin directly from the Greek. The copies he gave to 
Henrv VIII and to Cardinal VVolsey can be seen in 
the British Museum. Erasmus, commenting on Lin- 
acre's translations, said, "I present you with the 
works of Galen, by the help of Linacre, speaking better 
Latin than ever they spoke Greek." 

Galen's emphasis, in spite of his dissection of ani- 
mals, was not so much on the structures he found as 
on the contents of the cavities within them. Function, 
according to his doctrine, was mediated by humors 
which were respon.sible for all sensation, movement, 
desires and thought, and hence pathology was 
founded on humoral disturbance. The role of the 
organs of the body was to manufacture and process 
these humors. His teaching about the nervous system 
was that the blood, manufactured in the liver and 
carrying in it natural spirits, flowed to the heart where 
a change took place converting them into vital spirits. 
These travelled to the reie muahtle (the terminal 
branches of the carotid arteries at the base of the 
brain) where they were changed into animal spirits,- 
a subtle fluid which then flowed out to the body 
through hollow nerves. Some of the.se ideas Galen 
developed from those of his predece.s.sors (such as 
Alcmaeon, Herophilos, Erisistratos), some were 
inspired by his dissection of animals, but all were 
hypothetical, none had any experimental proof or 

1. Galen (130-200 A.D.). Opera Omnia (in acdibus Atdi el 
Andrea Asulani) (in Greek). Venice, 1525. 5 vol. 

2. Galen. Opera Omnia (in Greek). Basle, 1538. 

3. Galen. De Facullalibiis naturalibus, Latin translation by 
Thomas Linacre. London: Pynson, 1523; English transla- 
tion by A. J. Brock, Loeb Classical Library. London: 
Heineman, 1916. 

' The usage of the term animal spirits' throughout the 
centuries carries the connotation of the Latin anima meaning 
soul and has no reference to the modern meaning of the word 
'animal.' 

^ No other was to appear until the beginning of the eighteenth 
century when Johann Gottfried von Berger (1659-1736) 
published his textbook entitled P/iysiologa Medica sine natura 
humana. Wittenberg: Kreusig, 1701. 

' "Nor lesse Worthy of Commendation are the Cravings. . . 
those eleven pieces of Anatomic made for Andrea Vessalius 
design'd by Calcare the Fleming, an Excellent painter, and 
which were afterwards engraven in Copper by Valverdi in 
little." Evelyn, John. Sculpltira: or the History, and Art of Chalcog- 
raphy. London, 1662. The reference is to the plagiarism of the 
Spaniard, Juan Valverde. Vivae Imagines Partium Corporis 
Humani. Antwerp: Plantin, 1566. (His artist was Becerra.) 



even partial support, yet some of them were to last 
well into the nineteenth century. 

The sixteenth century gave to physiology its first 
textbook.^ This was the contribution of Jean Fernel, 
physician and scholar, who in 1542 published his 
De Naturali Parte Medicinae (4). This was so well 
received that it saw inany editions. In the ninth of 
these Fernel changed the title to Medicina (5) and 
named the first section of the revised book Physio- 
logia. According to Sherrington (6) this was the first 
use of the term 'physiology.' There is, however, a 
manuscript in the Danish Royal Library entitled 
Physiologus that deals with animals and inonsters. 
This copy is an Icelandic version of an apparently 
much-copied treatise; it is a kind of bestiary. For some 
time after Fernel's revival of it, the term 'physiology' 
was still used by most writers to mean natural philoso- 
phy. An example of this usage is to be found in the full 
title of Gilberd's book on the magnet published in 
1600. Although still grounded in a classification de- 
rived froin the four elements of the ancients, Fernel's 
physiology nevertheless shows dawning recognition 
of some of the automatic movements which we now 
know to be reflexly initiated for, although only the 
voluntary muscles were known to him, he realized that 
sometimes they moved independently of the will. 

Before the seventeenth century opened, a technical 
achievement in another field laid a foundation on 
which physiology was to spread. Lagging about 50 
years after the invention of printing came the develop- 
ment of copper plate engraving and accurate repro- 
ductions of anatomists' drawings became more 
widely distributed. Supreine, however, ainong the 
woodcuts contemporary with the early engravings 
were those made from the drawings of Jan Stephen of 
Calcar for the anatoinical studies of Vesalius (7^9). 
These, published in 1543, were to draw the praise of 
John Evelyn in his treatise on chalcography.^ After 

4. Fernel, Jean (1497- 1558). De Naturali Parte Medicinae. 
Paris: Simon de Colines, 1542. 

5. Fernel, J. Medicina. Paris: Wechsel, 1554. P/iysiologia, 
translated into French by Charles de Saint Germain, 
Les VII Livres de la Physiologic, composes en Latin par Messire 
Jean Fernel. Paris: Guignard, 1655. 

6. Sherrington, C. S. The Endeavour of Jean Fernel. Cam- 
bridge: Cambridge, 1946. 

7. Vesalius, Andreas (1514-1564). De Humani Corporis 
Fabrica. Basle: Oporinus, 1543; translated into English 
by J. B. de C. M. Saunders and C. D. OMallcy. New 
York: Schuman, 1947. 

8. Vesalius, A. Epitome. Basle: Oporinus, translated into 
English by L. R. Rind. New York: Macmillan, 1949. 

9. Vesalius, .\. Tabulae Sex. Venice, 1538. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



centuries in which human dissection could onl\' be 
done relatively furtively, a more liberal view had 
grown up in Italy and among a number of con- 
temporary anatomists, Vesalius is pre-eminent. In 
themselves, however, with the exception of an experi- 
ment showing that the nerve sheath is not vital for 
conduction, his studies made no contribution to the 
dynamics of function. Although an opponent of 
Galen and an exposer of his anatomical errors, 
Vesalius had no more satisfactory concept of nervous 
activity to offer than that of animal spirits flowing 
from the brain down pipe-like nerves to the muscles. 
Yet for the study of the nervous system, as for other 
branches of physiology, the publication of De Humam 
Corporis Fabrica is the outstanding contribution of 
the sixteenth century, the earlier chalk drawings of 
Leonardo Da Vinci (1452-1519) not being widely 
known to his contemporaries. The major contribu- 
tions of Vesalius were not in physiology but in anat- 
omy and in the demonstration that Galen was capa- 
ble of error (though he himself was not without error). 

At the opening of the seventeenth century the im- 
portant event for all science was the appearance 
(in 1600) of William Giiberd's* classic book De 
Magnete (10, 11). The significance of this work was 
not only as a landmark for the future of the physical 
sciences and of electrophy.siology through its dawning 
recognition of a difference between electricity and 
magnetism; it was the first book to advocate empirical 
methods and in this way heralded the scientific 
ferment of the eighteenth century. If one overlooks 
the last two chapters oi De Magnete, the book is revolu- 
tionary in its experimental approach. It stood out 
alone in an age when scholasticism was concerned 
with classification on qualitative lines without meas- 
urement and without validation. Authoritative state- 
ments of the ancients were the guides, and induction 
from experiment was virtually unknown. Gilberd's 
book makes a plea for "trustworthy experiments and 
demonstrated arguments" to replace "the probable 
guesses and opinions of the ordinary professors of 
philosophy." 

Gilberd was physician to Queen Elizabeth (whom 



he only just survi\-cdj and a sketch identified as a 
portrait of him appears in the contemporary draw- 
ing (now in the British Museum) made by William 
Camden, the Court Herald, of her funeral proces- 
sion in 1603. A contemporary oil portrait of him 
painted in 1591 has been lost and remains to us only 
in engravings. Gilberd was born and lived part of 
his life in his father's house in Colchester in East 
Anglia; a portion of this house still stands and, at 
the time of writing, is being restored. This flowering 
of the .scientific method came during the golden age 
of Elizabethan England; among Gilberd's contem- 
poraries were Shakespeare, Walter Raleigh, Philip 
Sydney, John Donne, Christopher Marlow and 
Francis Bacon. 

Francis Bacon has a place in the history of all 
.sciences, for he took scientific method a step farther, 
to observation he added induction and to inference he 
added verification. Scientists before him were content 
with performing an experiment in order to make 
an observation; from this oijservation a series of 
propositions would follow, each being derived from its 
predecessor, not by experiment but by logic. (Bacon 
somewhat unjustly criticizes Gilberd for proceeding 
in this way.) Bacon's contribution to scientific method 
was to urge, in addition, the rigorous application of 
a special kind of inductive reasoning proceeding 
from the accumulation of a number of particular 
facts to the demonstration of their interrelation 
and hence to a general conclusion. This was in- 
deed a new instrument, a Novum Organum (12). By its 
application he overthrew reliance on authority of 
the ancients and opened the way for planned experi- 
ment. Although he had no place in his method for 
the working hypothesis, and his forms of induction 
and deduction are scarcely those of the modern 
methodology, they were of considerable influence in 
its development. The intelligent lines of Bacon's 
face can be seen in his portraits. John Aubrey (13) 
tells us that he "had a delicate, lively hazel eie" and 
that "Dr. Harvey told me it was like the eie of a 
viper." 

The first major work in physiology exemplifying 



10. Gilberd, William (1540 (or 1544)- 1603). De Magnete, 
Magnetisque corporibus; et de mag?io magnete lellure; Physio- 
logica nova plurimis et argumentis et experimentis demonstrata. 
London: Peter Short, 1600; translated into English by 
the Gilbert Club, William Gilbert of Colchester, physician 
of London. London; Chiswick Press, igoo. 

11. Ibid. (2nd ed.) (posthumous). Gotzianio in Stettin, 1633. 
This book, far rarer than the first edition, carries more 
plates than the original, and has some additions by 
Wolfgang Lochmann of Pomerania (1594- 1643). 



12. Bacon, Francis (1561-1626). .Novum Organum. 1620; 
translated into English by Kitchin. Oxford, 1855. 

13. Aubrey, John (1626- 1697). Brief Lives set Down i66g- 
i6g6, edited by Andrew Clark. Clarendon Press, 1898, 

vol. 2. 



' The spelling of Gilberd's name follows the form seen on his 
portrait and memorial tablet; his name on his book is spelled 
Gilbert. 



HANDBOOK OF PHYSIOLOGY 



NEUROPH^'SIOLOGY 1 




FIG. I. Portrait of William Gilberd from an oil painting on wood, found by Silvanus P. Thompson 
in an antiquary's shop. The artist and the authenticity of the date on this portrait are unknown. 
The portrait is now in the possession of Miss Helen G. Thompson, by whose courtesy it is repro- 
duced here. The photograph of Tymperleys,' Gilberd's home at Colchester, was taken in 1957 
when the house was undergoing extensive restoration. A portion only of the house dates from Gil- 
berd's time. (Photograph by courtesy of Dr. G. Burniston Brown.) 



Bacon's methodolos;y was iioi on the nervous .system 
but on the circulation of the blood. Harvey's magnif- 
icent treatise De Motu Cordis (14) was a model for 
workers in all branches of physiology to follow. This 
small book (it has only 72 pages) was the first major 
treatment of a physiological subject in dynamic 
rather than static terins. By experiment Harvey dis- 
proved the Galenist doctrine that the motion of the 
blood in the arterial and venous systems was a tidal 
ebb and flow, independent except for .some leakage 
through 'pores' in the interventricular septum. By 
further designed experiments Harvey proved his own 
hypothesis "that the blood in the animal body is im- 
pelled in a circle, and is in a state of ceaseless motion." 
Harvey had advanced this hypothesis in 1616 but, 

14. H.^RVEV, VViLLI.aiM (i 578- 1 657). Exercttatin analomica de 
motu cordis et sanguinis in animiilihus. Frankfurt : Fitzeri, 
1628; translated into English by VVillius and Keys, Car- 
diac Classics, 1 94 1, p. 19. 

15. H.ARVEV, \V. Praelecliones anatomiae universalis. London: 
Churchill, 1886. (Reprint of Harvey's Lumleian lecture 
1616.) 



as a forerunner of modern scientific method, had 
proceeded to verify it before publishing his book. 
But even this triumph of the empirical method did 
not unseat in Harvey's thinking the belief in a soul 
located in the blood ('anima ipsa esse sanguis') (15). 
Harvey was Galenist enough to accept the rete mirabile 
as the destination of the blood within the craniitm, 
although doubt as to its existence in man had already 
been raised by Berengario da Carpi (16, i 7) a hun- 
dred years before. Harvey (18) had his own \'iews 
of nervous function. "I believe," he said, "that in the 
nerves there is no progression of spirits, but irradia- 
tion; and that the actions from which sensation and 

16. Berengario da Carpi, Giacomo (1470- 1550). Com- 
menlaria cum amplissimus addilionibus super analomia Mun- 
dtni. Bologna : Benedictis, 1 52 1 . 

17. Berengario da Carpi, G. Isagogae breves, perlucidae. In: 
Analomiam hurnani corporis, ad suorum scholasticorum preces 
in lucem edilae. Bologna, 1522; translated into English by 
H. Jackson, under the title A description of the Bnd\ of Mnn, 
being a practical Anatomy. London, 1664. 

18. Harvey, W. Praelecliones Analo?nwe Universalis, autotype 
reproduction edition. Philadelphia: Cole, 1886. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 





FIG. 2. Borclli and one of his sketches to show the center of 
gravity of man when carrying a load. (From BorelU, G.A. De 
Mntu Animalium, 2nd ed., Leydon : Gaesbeeck, 1685.) 

motion result are brought about as light is in air, 
perhaps as the flu.x and reflu.x of the sea." 

That nerves might play a role in the working of 
the heart as a mechanical pump was first suggested 
by Borelli the Neapolitan, professor of mathematics at 
Pisa and later at Florence, who applied the reasoning 
of his discipline to physiology and e\olved mechani- 
cal models for various bodily functions. His concept 
of the innervation of muscle was an initiation by the 
nervous fluid ('succus nervcus') of a fermentation in 
the mu.scle swelling it into contraction, for there were 
still many years to go before a dynamic concept of 
muscle was to emerge in spite of Harvey's demon- 
strations on the heart. Peripheral muscles were still 
regarded as passive structures rather like balloons to 
be inflated by nervous fluid or gaseous spirits reach- 
ing them through canals in the nerves. Borelli, by 
an ingenious experiment in which he submerged a 
struggling animal in water and then slit its muscles, 
demonstrated that the spirits could not Ije gaseous 
since no bubbles appeared in spite of the violent 
contractions. It was this experiment that led him to 
the suggestion of a liquid medium from the nerve, 
mixing in the muscle to cause a contraction by ex- 
plosi\e fermentation ('ebuUitio et displosio') (19). 

Giovanni Alphonso Borelli was a member of the 
group of experimental scientists banded together in 
the Accademia del Cimento under tlie patronage of 
the .science-loving Medici brothers in Florence. This 
small .scientific society, successor to the Lincei, existed 
for only a decade but was typical of the independent 

ig. Borelli, Giovanni .Alfonso (1608-1679). De molu ani- 
malium (pubUshed posthumously). Rome: Bernado, 1680- 
I ; a small section has been translated into English by 
Michael Foster. Lectures on the History of Physiology. Cam- 
bridge: Cambridge, 1901. 



groups centered on laboratory experiment that were 
to spring up in independence of the universities where 
the scholars had still not looked up from their books. 
Few as they were (there were only nine members) 
these laboratory scientists of the Accademia were to 
have a far-reaching though delayed influence on 
European thought, for in the final year of the acad- 
emy's existence they published their proceedings (20). 
Founded entirely on empirical methodology, this was 
a truly scientific text. It was, however, written in 
Italian although soon translated into English, and it 
did not reach the scientific world at large until Petrus 
van Musschenbroek of Leydeii made a Latin transla- 
tion (21). It was this book that, for example, influ- 
enced Stephen Hales so greatly in his experimental 
work. The volume included only one series on animal 
experimentation, but almost all the rest deals with 
the physics which are basic to the work a physiologist 
does in his laboratory. 

To his contemporary, Descartes, Borelli owed his 
application of mathematics to muscular action. This 
pungent philosopher, who rarely did an experiment, 
wrote a text that was to influence all experimenters, 
The Discourse on Method (22). It is not experimental 
method that he discusses, i)ut his own method of 
thought, his theory of knowledge." Scientists had 
just begun to look around them to olaserve nature 
and to let the statements about her by the ancients 
lie in the books when they had to meet a new and 
brilliant challenge; mathematics was the tool they 
were to use. Mathematics would not only elucidate 
the laboratory experiment but would provide the 
basis for an all-embracing theory of science. 

This great man bred in the gentle landscape of 
Touraine was to devote his life to a search for the 
truth, .seeking for himself a quiet environment for 
free thinking.' This he found for 25 years in the 

20. Saggi (It naturali esperienrji fattr nell Aecadeniui del Cimento, 
edited by L. Magalotti. Florence, 1667; translated into 
English by Richard Waller. Essayes of Natural Experiments 
made in the Accademie del Cimento. London, 1684. 

21. VAN Musschenbroek, Petrus (1692-1761). Testamina 
Experimentortum Naturaliuin captoruni in Accademia del Ci- 
mento. Leyden, 1731. 

22. Descartes, R. Discours de la Methode. 1637 ^ English trans- 
lation by E. S. Haldane and G. R. T. Ross. Philosophical 
Works of Descartes. Cambridge : Cambridge, 1 904. 

* "Methode de bien conduire sa raison, pour trouver la 
verite dans les sciences." 

' "Cum nil dignum apud homines scientia sua invenisset, 
eremum ut Democritus aliique vcri Philosophi elegit sibi 
juxta Egmundum in HoUandia, sibique solitarius in villula per 
25 annos remansit, admirandaque multa meditatione sua 
detexit" (Borel, p. 9). 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 




FIG. 3. Rene Descartes and his concept of the pineal gland. The photograph is from the portrait 
by Franz Hals in the Louvre, and the diagram is taken from de la Forge, Louis. Traili de I' Esprit 
de I'Homme, de ses Facultez, de ses Fonclions, et de son Union avec le Corps. SuiranI les principes de Mr. 
Descartes. Geneva : Bousquet, 1725. 



village of Egmond in liberal Holland, though even 
here he could not entirely escape lieing hounded by 
bigots. The mistake he made that the world regrets 
was to leave a milieu so congenial to his philosophic 
nature for the cold of Sweden and the exacting de- 
mands of Queen Christina. There, within a year, he 
died. His striking face with the intelligent eyes and 
quizzical eyebrow has been preserved for us in the 
fine portrait by Franz Hals that hangs in the Loux-re. 
A great man has many lives' written about him 
but those set down by his contemporaries usually 
have a special flavor. In the case of Descartes, the 
short account of his life and his philosophy written 
by Borel (23) (the inicroscopist) in 1669 gi\es one the 
feeling of bridging the centuries. Borel gives a list of 
the manuscripts found in Stockholm at Descartes's 
death in 1650, including the early treatise he wrote 
on music when he was only 22. Several of his letters 
were found, some of which Borel reproduces. The 
letters date from 1632 and give an intimate glimp.se of 
the struggle Descartes had to face in overcoming re- 
sistance to his theories among some of his con- 
temporaries. 

23. Borel, Pierre (1620- 1689). Vitae Renati Cartesii, Summt 
Philosophi Compendium. Frankfurt: Sigismund, 1676. 

* "It is an error to suppose the soul supplies the body with 
its heat and its movements." Passions de I'Ame, Article 5. 



Descartes (24, 25), having become convinced that 
in mathematics lay the tool for a unified theory of all 
science, had now to explain its role in physiology. It 
followed logically that the animal body and all its 
workings was a machine, the operation of this machine 
being directed from a control tower. In the brain with 
its bilateral development, the singly represented 
pineal body was chosen by Descartes to play this 
master role and (in man) it was given the added 
responsibility of housing the soul. In the concept of 
the body as a machine, energized not by an iniina- 
terial aninia* but by the external world impinging on 
it, lies a germ of the idea of reflex activity. 

To coming generations of neurophysiologists Des- 
cartes bequeathed the notion that impressions from 
the external world were conveyed by material animal 
spirits to the ventricles and there directed by the 
pineal gland into those outgoing tubular nerves that 
could carry them to the part of the body the subse- 
quent action of which would be the appropriate one. 
In animals this was presumed to be a purely mechani- 
cal action, but in man the soul, resident in the pineal, 
could have soine say in the direction taken by this 

24. Descartes, Rene (1596-1650). Passioru de fAnie. 
.-\msterdam, 1649. 

25. Descartes, R. De homine figuris, et latinate donatur a 
Florentio Schuyl, posthumous Latin version by Schuyl. 
Leyden: Moyardum & Leffen, 1662; first French edition, 
Traile de I'Homme, 1664; second French edition, 1677. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



central relay. Descartes recognized, however, that 
perhaps some of these actions lay outside the control 
of the will, citing as examples involuntary blinking 
and the withdrawal of the hand on burning. 

To neurophysiologists Descartes bequeathed an- 
other seed — what was later to be known as the re- 
ciprocal innervation of antagonist muscles. In order 
to ensure that while animal spirits were flowing 
into one set of muscles the opposing set should relax, 
he argued that the latter must have their supply of 
spirits blocked and he postulated that this must be 
eflTected by valves. Whether or not he was influenced 
in his thinking by Harvey's explanation of the 
valves of the veins is not known, although he was 
certainly aware of, and had commented on, Harvey's 
discoveries.^ Descartes was a member of what a sub- 
sequent irreverent generation was to call 'the bal- 
loonists.' Apparently unaware of Borelli's experiments, 
he thought the animal spirits to be "like a wind or a 
verv subtle flame" and that "when they flow into a 
muscle they cause it to become stiff and swollen, just 
as air in a balloon makes it hard and stretches the 
substance in which it is contained." 

A young contemporary of Descartes, though less 
directly influenced bs' him than was Borelli, was Wil- 
liam Croone who was working on muscle action. He 
too thought that the nervous 'juice' must interact in 
some way with the muscle (26). The "spiritous liquid" 
flowed in, mixed with "the nourishing juice of the 
muscle," and then the muscle "swell'd like a Bladder 
blown up. " Later (27} Croone was to modify this to 
a number of small bladders for each muscle fiber. 
Just as Borelli had been a founding member of a 
scientific society, so was Croone. He was one of the 
original group who in England formed the Royal 
Society, a society which unlike the Cimento has con- 
tinued to flourish and in which to this day eminent 
.scientists not only discuss but demonstrate their ex- 
periments before the members. The Royal Society 
has several distinguished lectureships, among which 
is the Croonian Lecture founded by the widow of 
William Croone. 

The Royal Society of London received its charter in 
1662, being founded for the promotion of "Natural 
Knowledge,' and it numbered among the founding 
members many who.se contributions are fundamental 

26. Croone, William (1633-1684). De raiione motus muscu- 
lorum (published anonymously). London: Hayes, 1664. 

■27. Croone, W. An Hypothesis of the Structure of the Muscle, and 
the Reason of its Contraction. Hooke's Philosophical Collec- 
tions, No. II. London, 1675. 



to physiolog)-. The mo\ing spirit was Robert Bovle, 
the 'father of chemistry' (whose first published work 
was, however, on Seraphick Love). Famous for his law 
(28) of gaseous pressures, he made his most directly 
physiological experiments on the respiration of ani- 
mals. It was still many years before physiologists were 
to elucidate the efTects of anoxia on the nervous 
system, and another hundred years were to pass before 
Priestley's and Lavoisier's work on oxygen, but Boyle, 
by using an ingenious compression chamber, demon- 
strated that air is essential for life. Almost unnoticed 
at the time, but since then perhaps overpraised, were 
the observations of John Mayow (29) on the chem- 
istry of respiration. His publication preceded (al- 
though his work was contemporary with) the some- 
what similar experiments of the Accademia del 
Cimento. 

In the early seventeenth century emphasis on the 
search for a chemical foundation for living phe- 
nomena characterized for the most part work in 
Holland and England in contrast to the physical and 
mathematical approach of the Italians and the 
French. The two contrasting .schools of thought were 
long to be known by the clumsy names of the iatro- 
chemical and iatromechanical schools. latrochemis- 
try, on the rather shaky foundations given to it by 
van Helmont (1577-1644) and by Sylvius (de La Boe) 
(1614-1672), provided the approach to the study of 
the nervous system of Thomas Willis, Sedleian Pro- 
fessor of Natural Philosophy at Oxford (30). Willis, 
whose clinical achievements outshone his scientific 
acumen, is recognized in neurology for his description 
of the circle of Willis and his dissection of the spinal 
accessory nerve. (Galen had identified only seven 
pairs of cranial nerves.) Willis was a close colleague at 
Oxford of Richard Lower, the Cornishman, champion 
of the theory that spirits flowing into the heart from 

28. Boyle, Robert (162 7-1 691). .\ew experiments physico- 
mechamcal, touching the spring of the air, and its effects, made, 
for the most part, in a new pneumatical engine. Oxford : VV. 
Hall, 1660. 

29. Mayow, John (1645-1679). Tractus Duo, quorum prior 
agit De Respiratione : alter De Radutiones. O.xford: Hall, 1668. 

30. Willis, Thomas (1621-1675). Cerebri anatome: cui accessit 
nervorum descriptio et usus. (Z)f systemate nervosa in genere"), 
illustrated by Sir Christopher Wren. London: Flesher, 
1664; translated into English by S. Pordage. London: 
Dring, Harper and Leigh, 1683. 



' Letter to Mersenne dated 1632, quoted in Oeuvres Com- 
pletes de Descartes, edition of Adam and Tannery, Paris; Cerf, 

1897-1910, vol. n, p. 127. 



8 



HANDBO(iK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



its nerves were what caused it to beat (31). Lower's 
more spectacular achievement was the apparent 
transfusion of blood, first in dog and then in man (32, 
33). We are surprised today that the man survived as 
long as he did, for the blood donor was a sheep. 

Thomas Willis had added to the prevalent Galenic 
ideas of nervous function the concept that the soul 
had two parts which he likened to a flame in the vital 
fluid of the blood and a light in the nervous juice. 
When they met in the muscle, they formed a highly 
explosive mixture which inflated the muscle. Yet even 
before the seventeenth century had run out, a voice 
was raised against such visionary explanations. Sten- 
sen (34)' '^he great Danish anatomist, writing from 
Florence in 1667, stated unequivocally that "Animal 
spirits, the more subtle part of the blood, the vapour 
of blood, and the juice of the ner\-es, these are names 
used by many, but they are mere words, meaning 
nothing." 

The seventeenth century, or grand siecle as it was 
known to Europe, had been gloriously opened by 
the De Magnete and gone on to the achievements of 
Galileo, Kepler, Huygens, Leibniz and Newton, and, 
although these were essentially achievements in 
mathematics, physics and astronomy, all branches of 
science were fermenting with the implications of these 
disco\'eries. The break with dogma was now more 
than a crack, though the Index Librorum Prolnb- 
itorutn fought a delaying action. The men of the 
arts were liberal in their championship of the scientists. 
John Milton's Areopagitica (35) is a clarion call for 
freedom of knowledge and distribution of books. 
Milton was a young contemporary of Galileo and 
went to see him in his old age. There is a poignancy 
about this visit to the old blind astronomer from the 
poet about to become blind. 

The students of the nervous system had the hardest 
fight against dogma for in their province lay the 

31. Lower, Richard (1631-1691). Tractatus de Corde item de 
Motu & Colore Sanguinis el Chyli cum Transitu. London: 
AUestry, i66g; English translation by K. J. Franklin. 
Early Science in Oxford. Oxford, 1932, \ol. g. 

32. Lower, R. The method observed in transfusing the blood 
out of one live animal into another. Phil. Trans, i : 353, 
1665-6. 

33. Lower, R. and E. King. An account of the experiment 
of transfusion, practised upon a man in London. P/ul. 
Trans. 2: 1557, 1667. 

34. Stensen, Nicholas (1638- 1686). Elernentorum myologiae 
.'Specimen. Florence: Stella, 1667, p. 83. 

35. Milton, John (1608-1674). Areopagitica. A speech for the 
Liberty of Unlicensed Printing to the Parliament of England. 
1644. 



structures most suspect as being the guardians of 
man's soul. But ranked behind them and influential on 
them were some of the greatest philosophers of their 
time. Prominent among the.se was Locke (36), the 
father of empiricism. Born in the West of England and 
trained as a physician, this man with his colorless 
personality and his clumsy prose was to channel the 
efforts of the next several generations of workers on 
the nervous system into a .search for the physiology 
of the mind. For his Essay on Humane Understanding 
he received immediate recognition and monetary 
reward, obtaining for it more than was paid to John 
Milton for Paradise Lost. 

Straddling like a colo.ssus the division between the 
seventeenth and eighteenth centuries is Newton, 
friend and correspondent of Locke, though to .scien- 
tists it is perhaps a bit disappointing to find that the 
subject of their correspondence was the interpretation 
of the New Testament (biblical history was a life-long 
interest of Newton). Newton's insight into the move- 
ment and forces of nature led him to make some 
tentative suggestions about the working of the nerv- 
ous system, and these were noted by the physiologists 
of the time. There is scarcely a single neurophysiolo- 
gist of the eighteenth century who does not explicitly 
attempt to align his findings with these conjectures 
of Newton. 

In the General Scholium (37) which he added to 
the second edition of the Principia (26 years after its 
first publication), Newton included a speculation. 
This was the idea of an all-pervading elastic aether 
"exceedingly more rare and subtle than the air," 
which he again suggested in the form of a question in 
the .series of Queries added to the second English edi- 
tion of his Opticks (38). Applying this suggestion to 
the nervous system, he said, "I suppose that the Capil- 
lamenta of the Nerves are each of them solid and 
uniform, that the \ibrating Motion of the Aetherial 
Medium may be propagated along them from one 
End to the other uniformly, and without interrup- 
tion. . . ." It is easy to understand how eagerly such a 
statement would be received by those who accepted 
the idea of a nervous principle running down the 
nerves but were worried that they knew of no fluid 
sufficiently swift and invisible. Newton's rather sketchy 
suggestion was therefore eagerly embraced by many 
of his contemporaries, one of whom, Bryan Robinson, 

36. Locke, John (163J-1704). An Essay concerning Humane 
Understanding. London: Holt, 1690. 

37. Newton, Isaac (1642-1727). Principia. London: 1687; 
edition with General .Scholium, 171 3. 

38. Newton, L Opticks (2nd ed., 24th Query). London: 1717. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



Regius Professor of Physic at the L'ni\ersity of Duljlin, 
even went so far as to claim that "Sir Isaac Newton 
discovered the Causes of Muscular Motion and Secre- 
tion" (39). 

At the opening of the eigthteenth century the sci- 
ence of the nervous system had reached diflferent levels 
in the various countries of Europe. In Germany in 
the first half of the century the Thirty Years War 
had brought science almost to a standstill, and in the 
fields of chemistry and physiology this stagnation de- 
veloped into a retrogression owing to the emergence 
of an extremely influential figure, Georg Ernst Stahl. 
In opposition to both the chemical and mathematical 
schools, Stahl set back the clock by the reintroduction 
of an immaterial anima which he held to be the sole 
activating principle of the body parts (40). The 
latter were regarded as having no dynamic properties 
of their own, being essentially passive structures. 
Since the search for an immaterial agent lies outside 
the scope of science, Stahl's doctrines, promulgated 
with arrogance and dogmatism, virtually extinguished 
experimental inquiry among his followers. Yet even 
writers sympathetic to his viewpoint granted that in 
attempting to follow his arguments one became "in- 
volved in a labyrinth of metaphysical subtlety" (41). 
The metaphysical approach of Stahl later came 
under criticism from Vicq d'Azyr (42) who suggested 
that the invention of an imaginary soul to resolve those 
phenomena that could not yet be explained by the 
laws of physics and chemistry was merely a cloak for 
ignorance, van Helmont did not escape the same 
criticism. 

In opposition to humoral or vitalistic concepts of 
nervous and muscular activity was a prominent 
champion of a 'solidist' theory, Giorgio Baglivi. This 
young man, whom Pope Innocent XII had appointed to 
be profes.sor of the theory of medicine and anatomy 
at Rome, put emphasis on the fibers of the muscles 
and the nerves, and so foreshadowed the importance 
that was to be given in the eighteenth century to the 
intrinsic structural properties of these tissues. He de- 

39. Robinson, Br\an (1680- 1754). A treatise of the Animal 
Oeconomy (3rd ed.). London: Innys, 1738. 

40. Stahl, Georg Ernst (1660-1734). Theoria Medica Vera 
Physiologiam et Pat/iologium, tanquam Doctrinae Medicae 
Partes veres Conternplativas e Naturae et Artis veris Junda- 
mentis. Halle, 1708. 

41. BosTOCK, John (1773-1846). Sketch of the History of Medi- 
cine from Its origin to the commencement of the nineteenth century. 
London: Sherwood, Gilbert & Piper, 1835. 

42. Vicq d'.'Kzvr, F. (1748-1794). Oeuvres de Vicq d'Azyr. 
Paris, 1805, vol. 4. 




FIG. 4. (iiorgio Bagli\i rising like a phoenix from the flames. 



veloped a theory (43) of an oscillatory movement of 
nerve fibers in order to account for both efferent and 
afferent activity and envisaged the dura mater as the 
source of these movements and the recipient of the 
returning oscillation. 

The leading medical center in Europe at this 
time was the University of Leiden. The empirical 
approach was urged by the physicist S'Gravesande 
(44) who advised that "It is Nature herself that 
should be examined as closely as possible . . . progress 
may be slow, but what we find will be certain." 
Petrus van Mmschenbroek (45), who had come to 
the Chair of Physics at Leiden from Utrecht in 1 740, 
had in a discourse on scientific method emphasized 
that physics should stand apart from metaphysics, 
that experimental analysis should antecede synthesis, 
that in the collection of evidence the exception should 
not be ignored, and that argument by analogy was 
fraught with danger. Yet it was essentially by analogy 
that the early eighteenth century viewed the func- 

43. Baglivi, Giorgio (1668-1707). De fibra motrice et 
morbosa. In: Opera Omnia. Leyden: Antonii Servant, 1733. 

44. S'Gravesande, Wilhelm Jacob (1688-1742). Physices 
Elementa Mathematica Experimenlis conjirmata sire Intro- 
ductio ad Philosopham .\ewtoniatinm. 2nd ed., 1725; 3rd 
ed., 1742,2 vols. Leiden. 

45. VAN MusscHENBROEK, Petrus (1692-1761). Discours a 
i' Organisation de V Experience. 1730. (His swansong as 
Rector at the University of Utrecht.) 



10 



HANDBOOK OF I'HVSIOLOGV ^ NEUROPHYSIOLOGS' I 




FIG. 5. Bocrhaave giving a class in botany. (From the en- 
graving by Jacob Folkema, reproduced by permission of the 
Rijksuniversiteit in Leiden.) 



tions of the nervous system; the brain was analogous 
to the heart and the nerves analogous to the arteries. 
In the one case the content was blood; in the other, 
nervous fluid. Some writers even spoke of "the systole 
of the brain . . . whereby the animal Juices are forci- 
bly driven into Fibres of the Nerves" (46). 

van Musschenbroek had been a pupil of Hermanii 
Boerhaave who came to the Chair of Medicine in 
Leiden in 1701. Boerhaave, essentially a chemist 
and a clinician, had an almost leaiendary fame as a 
teacher, which must, one feels, have been due to his 
personality, for he was not an experimenter and his 
doctrines were not at all progressive. He added little 
if anything new to the existing body of physiological 

46. Robinson, Nichol.a.s. A new system of /he Spleen, Vapours, 
and Hypoehondriak Melanchoh. London, 1729, p. '^62. 



knowledge. In his lectures (47, 48) on the nervous 
system he taught that "The Ventricles of the Brain 
have also many Uses or Ad\antages in Life, such as 
the perpetual Exhalation of a thin \'apour, or moist 
Dew." Himself a chemist, he made no experiments 
in ph\siology and was content to teach that "Tho' 
the nervous Juice or Spirits separated in the Brain 
are the most subtile and moveable of any Humour 
throughout the whole Body, yet are they formed 
like the rest from the same thicker Fluid the Blood, 
passing thro' many Degrees of Attenuation, till its 
Parts become small enough to pervade the last 
Series of Vessels in the Cortex, and then it becomes 
the subtile Fluid of the Brain and Nerves." His au- 
thority for this doctrine which he handed on to his 
eighteenth century pupils was the works of Galen 
who had died in 200 A.D. These teachings are difficult 
to reconcile with the exhortation expressed in his 
Aphorismi (49) that attention to facts and observations 
is the best means of promoting medical knowledge. 

Yet among his pupils Boerhaave numbered nearly 
all the prominent students of the nervous system in 
the eighteenth century: Haller, van Swieten, Monro, 
CuUen, de Haen, Pringle. His pre-eminence lay 
in the clinical field, and there can be no doubt that 
he had the greatest gift of a teacher, that of lighting 
the fire of enthusiasm in his students. It was two of 
them, Haller (50) and \an Swieten (51), who were 
responsible for the wider publication of his lectures, 
for on his own initiative he published little. 

van Swieten, who as a Catholic had little chance of 
advancement at the L'niversity of Leiden, went to 
Austria under the patronage of Maria Theresa and 
there founded the 'Old Vienna School,' patterning it 
on the medical clinic at Leiden. He was an advocate 
of a spare diet and active exertion and quoted in sup- 
port of his views "the case of a rich priest, who had 

47. BoERH.^.WE, Herm.-^nn (1668-1738). Instituliones Medicae 
in usus animal exercitationis domesticae. Leyden, 1708; anony- 
mous English translation. Academical Lectures on the Theory 
of Physic, being a genuine translation of his Institutes, and Ex- 
planatory Comment. London: Innys, 1743. 5 v'ol. 

48. BoERH.^.AVE, H. Praelectiones Academicae de .\lorbis Nervorum. 
Quas ex Audilorwn Manuscriptis collectas edi curavil. Jacobus 
van Eems. Leyden: van der Eyk & Pecker, 1761. 2 vol. 

49. BoERH.^AVE, H. .Aphorismi de cognoscendis et curandis 
morbis. Leyden: van der Linden, 1709. 

50. VON Haller, Albrecht (i 708-1 777). Commentarii ad 
Hermann Boerhaave Praelectiones Academicae in proprias 
Irutitutiones Rei Medicae. 1 739-1 744. 7 vol. 

51. VAN Swieten, Gerhard L.B. (1700-1772). Commentaria 
in Hermanni Boerhaave, aphorismos, de cognoscendis et curandis 
morbis. Leiden: Verbeek, 1742-1776. 6 vol. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY I I 




FIG. 6. Albrecht von Haller, the greatest physiologist of the eighteenth century, and de La Mettric 
whose treatise L'homme machine, addressed to Haller, caused a controversy that highlighted the ques- 
tion as to whether the soul lay in the province of the physiologist. The portrait of Haller is from the 
frontispiece of his Elementa Physiologiae and is an engraving by Tardieu; that of de La Mettrie is 
from an engraving in the Bibliotheque Nationale (reproduced here with permission), the original 
painting being a pastel by Maurice Quentin La Tour. 



enjoyed a fat living and long been a martyr to gout, 
chancing to be carried into slavery by a Barbary 
corsair, and kept for two years to hard labour and 
spare diet in the gallics lost his gout and his obesity 
together. ..." His master, Boerhaave, a martyr to 
gout, had died 34 years before, corpulence hastening 
his end. 

We have a contemporary description (52) of Boer- 
haave's habits and also of his looks. "He had a large 
head, short neck, florid complexion, light brown hair 
(for he did not wear a wig), and open countenance, 
and resembled Socrates in the flatness of his nose. ..." 
We are told that he ro.se at four in the inorning, but 
in the cold Dutch winters he allowed himself an e.xtra 
hour in bed before settling to work in his unhealed 
study. His chief relaxation was music and he played 
several instruments of which his favorite was the lute. 

It is at about this period — the middle of the eight- 
eenth century — that experimental work on the nerv- 
ous system began to be channeled into three main 
divisions: a) the elucidation of peripheral nerve 

52. Burton, William. An account of the Life and Writings of 
Hermann Boerhaave. London: Lintot, 1743. 



physiology and its differentiation from that of muscle, 
A) the recognition of the function of the spinal cord 
together with the development of ideas about reflex 
action, and c) the growth of knowledge about the 
brain as a neural structure unencumbered by dogma 
concerning the soul. 



EXCIT.iiiBILITY .AND TR.ANSMISSION IN NERVES 

In the field of physiology Boerhaave's most prom- 
inent pupil was Albrecht von Haller. Haller, a Swiss, 
was born in Berne and studied at Tubingen but was 
drawn to Leiden by the magnet of Boerhaave's 
teaching. After taking his medical degree he returned 
to .Switzerland where he divided his time between 
medicine, poetry and botany. In 1736 George II of 
England, Elector of Hanover, appointed him to the 
chair of the mixed sciences Anatomy, Surgery and 
Botany at Gottingen, a newly-founded university. 
It was here that Haller spent the experimental phase 
of his life as a scientist. 

Unlike his master Boerhaave, Haller was a great 
laboratory worker as well as a phenomenal scholar 



12 HANDBOOK OF PHYSIOLOGY -^ NEUROPHYSIOLOGY I 




FIG. 7. Two men whose ideas of irritability anteceded tiiose of Haller. Glisson's concept (1677) 
included a psychic stage between stimulus and contraction thereby differing from Haller's which 
postulated a purely peripheral reaction. Johannes de Gorters proposal of irritability based on 
mechanical movement was published in 1734. The portrait of Glisson is an engraving from the 
original painting in the Royal College of Physicians. That of de Gorter is photographed from an 
engraving, the generous gift of the Director of the National Museum of Science in I^eiden. The 
original painting was by J. M. Quinkhard, the artist of the portrait of van Musschenbroek repro- 
duced in figure 1 1 . 



and was the author of the most famous of the eight- 
eenth century textbooks of physiology, the Elementa 
Physiologiae (53). Although these volumes came into 
print after Haller's retirement to Berne, he had while 
teaching at Gottingen brought out his Primae Lineae 
Physiologiae (54) for, as he proceeded with his ana- 
tomical and experimental studies, his master's texts 
became less and less useful to him. In the preface to 
his own work he remarks that, since the time of Boer- 
haave, anatomy had developed so greatly as to be- 
come almost a new science. Haller had himself 
brought out an anatomy book (55) with fine engrav- 
ings, and anatomy was one of the four subjects on 
which he compiled bibliographies (56-59) that are a 



great source of information for the medical historian. 
They contain tens of thousands of references. 

For neurophysiologists Haller's most interesting 
work is his development of the concept of irritability. 
An earlier student of Boerhaave's at Leiden was 
Johannes de Gorter who later became physician to 
the Empress Elizabeth of Russia. He had in 1737 
published a volume (60) in which he brought out of 
obscurity the idea of the intrinsic irritability of tissues 
that had been postulated by Francis Glisson in the 
previous century. It is not clear whether de Gorter 
owed any of his ideas to Glisson. He mentions hiin 
only once (in De AIolii vitale, paragraph 58, p. 40) and 
this only in reference to the capsula hepatis. In any 



53. VON Haller, Albrecht (i 708-1 777). Elementa Physi- 
ologiae corporis humani. Lausanne: Marci-Michael Bous- 
quet et Soc, 1 757-1 765. 8 \ol. 

54. VON H.'iLLER, A. Primae lineae physiologiae in usiim praelec- 
tionium academicarium. Gottingen: Vandenhoeck, 1747. 

55. VON Haller, A. hones analomicae. Gottingen: Vanden- 
hoeck, 1 743-1 756. 

56. VON Haller, A. Bibliotheca Bolanica. Zurich: Orell, 1771- 
1772. 



57. VON Haller, A. Bibliotheca Chirurgica. Basle: Schweig- 
hauser, 1774; Berne: E. Haller, 1775. 

58. VON Haller, A. Bibliotheca Anatomica. Zurich: Orell, 

I774-I777- 

59. VON Haller, A. Bibliotheca medicinae practicae. Basle: 
Schweighauscr, 1776; Berne: E. Haller, 1778. 

60. DE Gorter, Johannes (1689-1762). Exercitaliones medicae 
quatuor. I: De motu vitale, 1734; il: Somno et vigilia; 
HI: De Jame; IV: De Siti. Amsterdam, 1737. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



13 




FIG. 8. Swammerdam's experiments including the one by which he proved that muscles were not 
swollen by an influx of nervous fluid when they contracted. Fig. V is of an experiment to show the 
change in shape of a muscle when stimulated by pinching its nerve. Fig. VI illustrates the pulling 
together of the pins holding the tendons when the muscle contracts. Fig. VIII is the crucial one in 
which a drop of water is imprisoned in the narrow tube projecting from the vessel enclosing the 
muscle. Swammerdam found that when he stimulated the nerve by pulling it down by a wire, the 
muscle contracted but the drop of water did not move. He concluded that the volume of the muscle 
did not expand on contraction. It is the fact that the wire was made of silver (filium argenteum) 
and the loop of copper (filium aeneum) that has credited Swammerdam with the use of bimetallic 
electricity as a stimulus to nerve. Some authors however interpret the action in this experiment as 
the mechanical pull on the nerve. Some originals of Swammerdam's plates can be seen at the 
National Museum of the History of Science in Leiden. (From Biblia .Naturae. Amsterdam, 1 738). 



case his concept of intrinsic irritability differed from 
that of GHsson in being part of a dynamic scheme in 
which inovements of muscles and nerves acted me- 
chanically on each other (61). Glisson (62) had been 
among the few scientists of the seventeenth century to 
test experimentally the Galenist doctrine that muscu- 
lar contraction was due to an inflow of nervous fluid 
inflating the muscle. He had demonstrated by immer- 
sion of a inan's arm in water that the level did not 
rise on contraction. Swammerdam,'" in Holland, 
reached the same conclusion from experiments on 
frogs (fig. 8). From such experiments, Glisson had 
gone on to develop a concept of intrinsic irritability 
varying in kind for the different nervous functions. 
As Regius Professor of Physic at Cambridge, Glisson 

61. DE GoRTER, J. Exercilaliones Medico Qiiinta V: De aclione 
viventium particulari. Amsterdam, 1 748. 

62. Glisson, Francis (1597^1677). Traciatus de venlricuto el 
inleslinis. London: Henry Brome, 1677. 



was to a certain extent bound by the statutes goxern- 
ing these professorships to teach the doctrines of 
Hippocrates and of Galen, and this may have limited 
him in the development of this new idea of irrita- 
bility. 

In Haller's hands the idea blossomed into a concept 
that was to dominate physiology for over a century. 
His theory differed from Glisson's in that he omitted 
the intermediate element of psychic perception be- 
tween the irritation and the contraction. The first 
expression of his theory of the relationship of con- 
tractility to irritability is found in 1 739 in his com- 
mentaries on Boerhaave's lectures and a fuller de- 
velopment in his Elementa Physiologiae, but it is in his 

"No known portrait of Swammerdam exists. In the nine- 
teenth century a publisher took one of the heads from Rem- 
brandt's Anatomy Lesson and put out a lithograph w'nich he 
labelled with Swammerdam's name. This was a stroke of 
imagination rather than fact. 



H 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



Gottingen lectures (63) gi\cn in 1752 (and published 
the following year) that the concept is most full\- de- 
veloped and supported by experimentation. Haller's 
own definitions for the dual properties of irritability 
and sensibility were as follows: "I call that part of the 
human body irritable, which becomes shorter on 
being touched; very irritable if it contracts upon 
slight touch, and the contrary if by a violent touch it 
contracts but little. I call that a sensible part of the 
human body, which on being touched transmits the 
impression of it to the soul; and in brutes, in whom the 
existence of a soul is not so clear, I call those parts 
sensible, the Irritation of which occasions evident 
signs of pain and disquiet in the animal." 

One sees immediately the bogey of the early physi- 
ologists raising its head — the necessity, on invoking 
the soul, for differentiating processes in man from 
those in animals. Haller describes his technique for 
determining sensibility as follows: "I took living ani- 
mals of different kind, and different ages, and after 
laying bare that part which I wanted to examine, I 
waited till the animal ceased to struggle or complain, 
after which I irritated the part, by blowing, heat, 
spirit of wine, the scalpel, lapis infinalis, oil of vine- 
gar, and bitter antimony. I examined attentively, 
whether upon touching, cutting, burning, or lacerat- 
ing the part, the animal seemed disquieted, made a 
noise, struggled, or pulled back the wounded limb, 
if the part was convulsed, or if nothing of all this 
happened." 

Haller recognized that nerves arc "the source of all 
sensibility," but applied his dichotomy of irritability 
and sensibility to various types of nerves, noting that 
all nerves are not irritable according to his definition 
(with its insistence on resultant contraction). He thus 
approached the differentiation of motor and sensory 
nerves. Still incorporated in his hypothesis was the 
1600-year-old concept of a nervous fluid within the 
nerves. It might be thought that once the microscope 
had been invented, the question of whether or not 
the nerves were hollow pipes might have been 
quickly settled. Indeed in 1674 Leeuwenhoek (64), 
with the limited magnification of his simple micro- 

63. VON Haller, A. De paitibus corporis humani scnsibilibus 
et irritabilibus. Comment. Soc. reg. Set. Gottingen 2: 114, 
1753; English translation by M. Tissot, M.D. A disserta- 
tion on the sensible and irritable parts oj animals^ from a 
treatise published in the Transactions of the Royal Society 
of Gottingen and read in the .'\cademy of Gottingen by 
Haller on April 22, 1752. Printed by J. Nourse at the 
Lamb opposite Katherine-street in the Strand, 1755. 

64. VAN Leeuwenhoek, .■\ntonj (1632-1723). Phil. Trans. 9: 
178, 1674. 



scope, had specifically searched for cavities in the 
nerves of a cow but his results were equivocal. One 
hundred years later this issue was still unresolved. 

The only competing hypothesis, which received 
but little support, was that the nerves were cords 
that communicated sensation to the brain by their 
\ibrations (rejected by Boerhaave as "repugnant to 
the Nature of the soft, pulpy and flaccid nerves"). 
This view was also rejected by Haller. 

In considering how a fluid could possibly flow as 
swiftly as nerves can be observed to act, Haller pro- 
posed that it must indeed be a very subtle fluid imper- 
ceptible to the eye yet more substantial than heat, 
aether, electricity or magnetism. In another comment 
he granted that electricity was a most powerful stimu- 
lus to nerves but that he thought it improbable that 
the natural stimulus was electrical. Thinking always 
in terms of electricity flowing as down a wire, Haller, 
like so many physiologists after him, felt the lack of 
insulation around the nerve to be a critical argument 
against nervous influence being electrical. 

However, the notion of electricity as a transmitter 
of nervous acti\ity kept cropping up at about this 
time. Alexander Monro (65), Professor of Medicine 
and Anatomy in the University of Edinburgh, a 
pupil of Boerhaave and first of the great dynasty of 
Monros, pointed out that no cavities could be seen in 
nerves, that no drops of fluid came out when a nerve 
was cut, and that the nerve did not swell when ligated; 
and he rather cautiously skirted the possibility of 
electricity being the agent. But he too considered it 
only in terms of electricity running down a wire and, 
like Haller, was bothered that the nerve was inade- 
quately insulated to prevent loss. "We are not suffi- 
ciently acquainted," he said, "with the properties of 
aether or electrical effluvia pervading everything, to 
apply them justly in the aniitial oeconomy; and it is 
difficult to conceive how they should be retained or 
conducted in a long nervous cord." 

Electricity had also been suggested by Stephen 
Hales (66) in refuting a suggestion that the swelling 
of muscles was due to inflow of blood. This country 
clergyman, without formal scientific or medical train- 
ing, by his experimental skill and keen observation 
became one of the outstanding contributors to knowl- 
edge of the circulation. In writing of the nerves he 
said, "From this very small Force of the arterial Blood 

65. Monro, .-Xlexander (1697-1762). The works of Alexander 
Monro (collected by his son). Edinburgh: Charles Eliot, 
1781. 

66. Hales, Stephen (1677-1761). Statical Essays. London: 
Innys and Manby, vol. I, 1726; vol. II, 1732. 




THE HISTORICAL DEVELOPMENT OF NEUROPHVSIOLOG V I 5 



^f^'l 



FIG. 9. Thf Abbe Nollt-t and some of his experiments in which he electrified plants and animals. 
The portrait, which shows the Abbe in his study at La Mouette, is from the oil painting by Jacques 
de Lajoue that hangs in the Musee Carnavalet in Paris and is reproduced by kind permission of the 
Conservateur, M. Charageat. The illustration on the right is from Nollet's book, Recherches sur les 
Causes Particuliers des Phenomhies Electnques. Paris, I 749. 



among the muscular Fibres we may with good reason 
conclude, how short this Force is of producing so 
great an Effect, as that of muscular Motion, which 
wonderful and hitherto inexplicable Mystery of Na- 
ture, must therefore be owing to some more vigorous 
and active Energy, whose Force is regulated by the 
Nerves; but whether it he confined in Canals within 
the Nerves, or acts along their surfaces like electrical 
Powers, is not easy to determine." 

At the end of the century came Galvani. His 
famous Commentary, published first in 1791, appeared 
at a time of intense interest in electricity. The demon- 
stration by Stephen Gray (67) in England that the 
human body could be electrified had been taken up 
and popularized by the Abbe Nollet (68) at the 



French Court and by Hausen (69), the Professor of 
Mathematics in Leipzig. Each had copied Gray's ex- 
periment in which he suspended a boy by ropes from 
the ceiling, bringing a flint-glass tube that had been 
charged by friction close to his feet and watching the 
attraction of a leaf-brass electroscope to his nose (see 
fig. 10). 

Electroscopes of this primitixe type were the only 
instruments then available for the detection of elec- 
tricity, the most sensitive one being that developed by 
the curate of a rural parish in Derbyshire (70). This 
delicate instrument with its gold leaves was identified 
by his name as Bennet's electrometer, though it was 
.scarcely a metrical device. Sources of electricity were 
still the frictional machines, first globes of sulphur, 
gla.ss or porcelain, and later revoking discs. It was 



67. Gray, Stephen (?-i736). Experiments concerning elec- 
tricity. Phil. Trans. 37: 18, 1731. 

68 Noi,i.ET, Jean-Antoine (1700-1770). Essai sur I'electricile 
des corps. Paris: Guerin, 1746. 



69. Hausen, Christian August (1693-1743). Novi projeclus in 
historia electricitatis. Leipzig, I743' 

70. Bennet, Abraham (1750- 1799). .New Experiments on Elec- 
tricity. Derby : John Drewry, I 789. 



1 6 HANDBOOK OF PHYSIOLOGY -^ NEUROPHYSIOLOGY I 




FIG. lo. The experiment of electrifying a boy, from the French translation of the book by F. H. 
VVinckler (Professor of Greek and Latin at Leipzig) entitled, Essai sur la Nature. Les ejjets el les causes 
avec description de deux nouvelles machines a Eleclricite. Paris: Jorry, 1748. (Photographed from the 
copy in the Wheatland collection by kind permission of Mr. David Wheatland.) 




vw,. I I. \ an Mnsschciiljroek and a Leyden jar. The portrait 
is from the oil painting by J. M. Quinkhard which hangs in the 
Museum of the History of Science in Leiden. The jar is an 
early one, rather large in size, also from the same museum, 
by the courtesy of which these photographs are reproduced. 



not until the development of the Leyden jar by Petrus 
van Musschenbroek, Professor of Physics in Leiden, 
that physiologists gained a much more stable and 
powerful source of electricity. 



van Musschenbroek, striving to conserve elec- 
tricity in a conductor and to delay the loss of its 
charge in air, attempted to use water as the con- 
ductor, insulating it from air in a nonconducting glass 
jar. However, when he charged the water through a 
wire leading from an electrical machine, he found the 
electricity dissipated as quickly as e\er. His assistant, 
Andreas Cuneus, while holding a jar containing 
charged water, accidentally touched the inserted wire 
with his other hand and got a frightening shock. W'ith 
one hand he had formed one 'plate,' the charged 
water being the other, and the glass jar the inter- 
\ening dielectric. A condenser was born. On touching 
the wire with his other hand he had shorted this 
condenser through his body giving himself such a 
jolt that he thought "his end had come" Q]i). van 
Musschenbroek wrote to Reamur describing a similar 
experience. Storage of electricity had now become 
pos.sible and in fact had been achieved independently 
by almost the same means (an electrified nail dipping 
into a vial containing liquid) by von Kleist (72) of 

71. Quoted in J. -A. Nollet. Metnoire de l' Academic Royale de 
Sciences. Paris, 1746, p. 1-25. 

72. VON Kleist, Ewald Juroen (d. 1748). Letter to J. G. 
Kriigcr, quoted in Geschichte der Erde Halle 1746, p. 177; 
and letter to Winkler (J. H. Winkler. Die Eigenschaften 
der electrischen Materie und des electrischen Feuers. Leipzig, 
■ 745)- 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



Kamin in Pomerania, yet another of the indefatigable 
company of eighteenth century clergymen to whom 
science owes so much. 

Both the electroscope and the Lcsclen jar were 
used by Galvani in the experiments he had begun 
not later than 1 780. He was also familiar with the 
fact that some animal forms, notably the marine tor- 
pedo and the electric eel, had intrinsic electricity. 
Scientific studies of this type of animal electricity 
had begun with the work of John Walsh (73) in 1733 
and have continued to this day. In those days the 
production of a spark was considered a sine qua non 
for full acceptance of the electrical nature of a 
phenomenon; this was lacking for the fish until after 
Gahani's time when Matteucci developed a tech- 
nique for demonstrating it (see fig. 12). For many 
years before Galvani's day, as demonstrated for 
example by Swammerdam and by the French anato- 
mist Joseph Guichard Duverney,^^ it had been known 
that the limbs of a frog could be convulsed by me- 
chanical irritation, and electricity applied directly to 
the muscle already had been used by many phy.sicians 
(and quacks) to animate paralytics. 

The three chief observations that stand out from 
the many experiments reported by Galvani in his 
original Commentarius (74) were a) that a frog's nerve 
muscle preparation, although at a distance from a 
sparking electrostatic machine, would twitch when 
touched by an observer (in the light of later knowledge 
this was called induction at a distance, with stimula- 
tion occurring by the 'returning stroke' at the moment 
of sparking); *) that atmospheric electricity could be 
used to stimulate frogs' legs if a long wire were erected 
(the principle of the lightning conductor); and c) that 
frogs' legs twitched when hung by brass hooks to an 
iron railing even in the absence of a thunderstorm. 
This last, the most important discovery in his first set 
of experiments, was due to the current that flows be- 
tween dissimilar metals when connected in a circuit, 
though Galvani did not understand this at the time 
and attempted to explain all his results as the presence 
of intrinsic animal electricity. 

The Commentartus was reprinted three times, twice 
in 1 791 and again in the turimlent year i 792 (the year 
that France seized Savoy); then it reached scientists 

73. Walsh, John (1725-1795). On the electric property of 
the torpedo. Phil. Trans. 63: 461, 1773. 

74. Galvani, A. (1737- 1798). De viribus electricitatis in 
motu musculari. Comment ar his De Bononiensi Scientiarum el 
Artium Insliluto alque Academic Commenlarii 7: 363, 1 791; 
English translation of 2nd reprinting of Galvani's Com- 
mentary by M. G. Foley. In: Galvani: Effects of Electricity 
on Muscular Motion. Norwalk: Burndy Library, 1954. 




FIG. 12. Galvani and the experiment on muscle contraction 
in the absence of any metals. The portrait is from the contem- 
porary oil painting in the Library of the University of Bologna 
(reproduced by courtesy of Dr. G. Pupilli). The experiment 
in which one leg is being stimulated by touching the nerves 
from the severed spinal column is reproduced from Aldini's 
book, Essai sur Ic Galvanisme. Paris: Piranesi, 1804. 



outside Italy. Through the great controversy stirred 
up by Volta which continued after Galvani's death in 
1798 (Galvani's less prudent nephew Aldini cham- 
pioning his cause), two extremely important areas of 
knowledge developed from the original observations. 
One was the recognition and elucidation of the 
electrical properties of mu.scle and ner\'e which were 
to lead directly to the discovery (by du Bois-Reymond 
in the next century) of the action potential of nerve, 
and the other was the developinent (by Volta) of bi- 
metallic electricity into the electric battery, one of 
the major technological steps in the history of science. 
Volta had striven to explain all the frog experiments 
by bimetallic currents, insisting that to produce elec- 
tricity three substances were always necessary, two 
heterogeneous metals and a third conducting material 



" This, one of the early public demonstrations of the stimula- 
tion of muscle through irritation of its nerve, was made before 
the Academic Royalc de Sciences in Paris in 1700, and is 
reported for that year as follows: "M. Du Verney shewed a frog 
just dead, which in taking the nerves of the belly of this animal 
which go to the thighs and legs, and irritating them a little 
with a scalpel, trembled and suffered a sort of convulsion. 
Afterwards he cut these nerves in the belly, and holding them 
a little stretched with his hand, he made them do so again by 
the same motion of the scalpel. If the frog has been longer dead 
this would not have happened, in all probability there yet 
remained some liquor in these nerves, the undulation of which 
caused the trembling of the parts where they corresponded, 
and consequently the nerves are only pipes, the effect whereof 
depends upon the liquor which they contain." History and 
Memoirs of the Roy. Acad. Sci. Paris. Translated and abridged by 
John Martyn and Ephraim Chambers. London: Knapton, 
1742, p. 187. 



i8 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 




FIG. 13. Volta and the experiment cf Galvani that led to the development of the Voltaic pile. 
The engraving of Volta is from the drawing by Roberto Focasi. Volta was an admirer of and was 
honored by Napoleon, one of whose gestures he seems to have caught. Behind him is a Voltaic pile. 
The sketch at the right was composed by an artist from a drawing made by du Bois-Rcymond when 
he visited Galvani's house 54 years after the latter's death. It depicts the experiment (designed to 
test atmospheric electricity) in which Galvani stumbled on the phenomenon of bimetallic electricity. 
(From Reden von Emit du Bois-Reymond, 1887, vol. 2.) 



to complete the circuit. If thi.s third material were a 
frog's muscle, it would by \irtue of its irritability react 
to the flow of bimetallic electricity, but its role (ac- 
cording to Volta) was solely that of an electroscope 
(75). When Aldini (76) demonstrated by dipping 
ends of nerve and muscle in mercury that the same 
effect could be obtained with a single metal, Volta 
replied that the .surface in contact with the air 
suffered a change that made it heterogeneous with 
the depth. This tortuous argument was disproved by 
von Humboldt (77). 

Before Galvani's death an anonymous (78) tract 
was published, almost certainly with his collaboration, 
in which an experiment was described on the twitch- 
ing of muscles in the absence of any metals or external 

75. Volta, Alessandro (1745-1827). On electricity e.xcited 
by the mere contact of conducting substances of different 
kinds. Phil. Trans, go: 403, 1800. 

76. Aldini, Giovanni (1762- 1834). De animali Electricitate 
dissertationes dime. Bologna, 1794. 

77. VON Humboldt, Frederick Alexander (1769-1859). Ver- 
suche iiber die gereizle Muskel- und .Nervenjasser. Posen und 
Berlin, 1797. 

78. Anonymous. Dell'uso e dell' attivita dell'Arco condultore nelle 
contrazioni del muscoli. With Supplemento. Bologna: S. 
Tommaso Aquino, 1 794; part of the Supplemento has 
been translated into English by M. Tschou in: B. Dibner. 
Galvani-Volta. Nor walk: Burndy Library, 1952. 



source of electricity. A contraction was demonstrated 
when the cut end of a frog's spine fell over onto its 
muscle or when one limb was drawn up to touch the 
exposed sciatic nerve (see fig. 12). In this case the 
source of electricity was what we now recognize as 
the current of injury. Even after this demonstration 
(79) Volta tried to explain the current flow as the re- 
sult of heterogeneity of tissues (muscle and nerve). 

The design of Humboldt's experiments and the 
clarity of his reasoning are a pleasure to study in 
the welter of acrimonious controversy that greeted 
GaKani's findings. Without bias towards either 
protagonist Humboldt repeated their experiments, 
examined their interpretations, designed new experi- 
ments to test their hypotheses and came to the con- 
clusion that Galvani uncovered two genuine phe- 
nomena (bimetallic electricity and intrin.sic animal 
electricity) and that the.se were not mutually exclu- 
sive. Humboldt demonstrated that both great scientists 
erred in their interpretations of their experiments; 
however, from these were to grow the science of 
electrophysiology on the one hand and, on the other, 
the brilliant development of the electric battery. Not 
only does Humboldt expose the erroneous parts of 
Galvani's and of Volta's interpretations but also 

79. Volta, .\. Phil. Mag. 4: 163, 1799. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



those of the writers who rushed in so precipitately to 
take up arms for one or the other protagonist — Pfaff 
(80), Fowler (81), Valli (82), Schmuck (83), each re- 
ceived his rebuke. He tells us that he thought some 
of the problems out while sitting at the foot of Mt. 
Bernard reading de Saussures' Voyages dans les Alpes 
(84). Humboldt was a great traveller (especially at a 
period when he was an inspector of mines) but did 
not let this interfere with his experiments, for he took 
his apparatus along with him, even on horseback. 

The pursuit of research in animal electricity was 
carried on in many countries, the most valuable contri- 
butions coming first from the Italian .scientists. Their 
task was made easier for them by Oersted's discovery 
of electromagnetism and its de\elopment by Nobili 
into a useful form of galvanometer. Oddly enough 
Oersted's researches (85, 86) that led to his important 
experimental demonstration of the relationship be- 
tween electricity and magnetism were motivated by a 
metaphy.sical belief in the universality of nature, a 
faith inspired by his adherence to Natiirphilosophie. 
This romantic doctrine with its facade of facts was 
very powerful in Germany from about 1810 to 1840 
and was derived from Kant's rejection of empiricism 
and his philosophy of universal laws known a priori 
by intuition. Oersted's own a priori belief was so 
strong that he did not hesitate to make his first experi- 
mental test of it in the classroom during a lecture to 
advanced students at the University of Copenhagen. 
The experiment worked; when current flowed in a 
single loop of bent wire, a magnet below it moved. This 
great discovery led to the development of instruments 
with multiple windings and to moving coil galvanom- 
eters. The contribution of Nobili, Professor of Physics 

80. Pfaff, Chrktophe-Henri (1773-1858). ,\bhandlung 
liber die sogennante thierische Electrizitat. Gren's J. 
Physik. 8(2): 196, 1798. 

81. Fowler, Richard. Experiments and observations relative to 
the influence lately discovered by M. Galvani, and (oninionly 
called Animal Electricity. Edinburgh: Duncan, 1793. 

82. Valli, Eusebe (1755-1816). Experiments in Animal Elec- 
tricity. London: Johnson, 1793- 

83. ScHMiJCK, Edmund Joseph. Beitrdge zur neuern Krnntniss 
der thierische Elektricitdt. Mannheim, 1792. 

84. DE Saussures, H. B. Voyages dans les Alpes. Neuchatel, 1796. 

85. Oersted, Hans Christian Ci777^i850- Experiences sur 
un effet que le courant de la pile e.xcite dans I'aiguille 
aimantee. J. Phys. Chim. 91: 72, 1820: English translation 
in Ann. Phil. 16: 273, 1820. The earliest announcement of 
Oersted's discovery was in a four-page pamphlet (now 
rare) entitled Experimenta circa efectum conflictus electrici 
in acum magneticum. Copenhagen, 1820 (copy in the 
Wheeler collection, New York). 

86. Oersted, H. C. Galvanic magnetism. Phil. .\iag. 56: 394, 
1820. 




FIG. 14. Matteucci and two of his experimental procedures. 
The portrait is reproduced from the old yellowing photograph 
in the Schola Normale .Superiore in Pisa (by courtesy of Dr. G. 
Moruzzi). .-Xbove on the right is Malteucci"s illustration of his 
rheoscopic frog, and below is his experiment demonstrating 
that the discharge of a marine torpedo can make a spark cross 
a gap. 



and Natural History at Florence, was the astatic 
galvanometer (87) in which two coils of wire wound in 
opposite directions cancelled the effect of the earth's 
own magnetism. 

It was Matteucci, the Professor of Physics at Pisa, 
who laid the groundwork of muscle electrophysiology 
that was to be developed so exhaustively by du Bois- 
Reymond. Carlo Matteucci (88) was one of the 
prominent figures in the Risorgimento. A great liberal 
and a great patriot, he attempted to coordinate the 
efforts of all European liberals when the 1 848 revolu- 
tion broke out. When Italy was united in 1859, he 
was made a Senator. He was one of the early Min- 
isters for Public Instruction in Italy. His contribu- 
tions have never received adequate recognition, mainly 
owing to the acrimonious attacks made on his work 
by du Bois-Reymond who came near to diminishing 
his own stature by his sour polemics. Matteucci had 
rai.sed the question as to where in the nerve-muscle 

87. Nobili, C. Leopold (1784-1835). Uber einen neuen 
Galvanometer. J. Chem. u. Phys. 45: 249, 1825. 

88. Matteucci, Carlo (1811-1865). Leqons sur les Phe- 
nomenes Physiques des Corps Vivanls, translated by Clet. 
Paris: Masson, 1847; English translation by Jonathan 
Periera. Lectures on the physical phenomena of living beings. 
Philadelphia: Lea and Blanchard, 1848. 



20 



HANDBOOK OF PPn'SIOI.OGV 



NEUROPHYSIOLOGY I 




FIG. 15. Johannes Miiller and his famous pupil von Hehn- 
holtz. The deHcate chalk drawing of Miiller was at one time 
in the Surgeon General's Library (now the National Library 
of Medicine). The picture of von Helmholtz shows him as a 
young man in the period when he made his major contribu- 
tions to the physiology of peripheral nerve. 



preparation the electricity lay and had thought that 
muscle alone could produce it. The preparation 
used by Matteucci was a frog's leg complete be- 
low the knee with only the isolated nerve abo\e 
it. Galvani's frogs retained a piece of the vertebral 
column with the insertion of the nerve into its 
portion of the spinal cord. Matteucci's contribu- 
tions in brief were a) the galvanometric detection of 
a current flow between the cut surface of a muscle 
and its undamaged surface, demonstrated in Ijoth 
animal and man (89, 90); h) the multiplication of 
current by serial arrangement of cut muscles so that 
the transverse section of each touched the longitudinal 
section of the next; c) the decrease in this current dur- 
ing tetanus caused by strychnine (90) (the germ of the 

89. Matteucci, C. Sur le courant electrique de la grenouille. 
Ann. chim. et phys. 68: 93, 1838. 

90. M.\TTEUcci, C. Deu.Kieme memoire sur le courant elec- 
trique propre de la grenouille et des animau.x a sang 
chaud. Ann. chim et phys. 80: 301, 1842. 



" ". . . while each organ of sense is provided with a capacity 
of receiving certain changes to be played upon it, as it were, 
yet each is utterly incapable of receiving the impression 
destined for another organ of sensation." Quoted from Bell, 
Charles (1774-1842). Idea of a new anatomy of the brain, submitted 
for the observation of his friends. Privately printed, 1811. 

""It is more probable that every nerve so affected as to 
communicate sensation, in whatever psirt of the nerve the 
impression is made, always gives the same sensation as if 
affected at the common seat of the sensation of that particular 
nerve. ..." Quoted in The Works of John Hunter edited by J. F. 
Palmer. London; Longmans, 1835. 4 vol. 



discovery of the action current); and d) the aljility 
of a frog's mu.scle contraction to generate enough 
electricity to stimulate the nerve of another nerve- 
muscle preparation when laid across it (the rheo- 
scopic frog) (91, 92). Matteucci was inconsistent in 
his interpretation of this finding and showed his 
characteristic vacillation between an explanation in 
terms of electricity and one based on nervous force. 
He named the effect the 'secondary contraction.' 
Matteucci (93) also noted such important laijoratory 
phenomena as the difference in stimulating effect of 
make' and 'break' shock.s, and the polarizing effects 
of prolonged flow of current on electrodes. He noted 
that polarization could occur inside the muscle and 
thus laid the ground for all the work that was to 
follow on polarization and electrotonus. 

du Bois-Reymond, of French name and Swiss 
descent, lived all of his working life in Berlin. He was 
a pupil of the greatest physiologist of the time, 
Johannes Miiller. Miiller, professor first at Bonn and 
then at Berlin, was a gifted teacher who could count 
among his pupils von Helmholtz, von Briicke and 
Sechenov. His Handbuch der Physiologie (94) was the 
great textbook of the nineteenth century, and the 
journal he founded, Mtiller's Archives fiir Analomie und 
Physiologie, as a successor to Reil's first physiological 
journal, was the main outlet for the stream of research 
that was coming from the German schools at that time. 
His own interests lay mostly in sensory physiology 
where his name is always associated with the "Law of 
.Specific Nerve Energies,' although this concept in 
fragmentary form had certainly occurred to others 
before him, including notably Charles Bell''- and John 
Hunter.'-' By this law Miiller formulated the findings 
that wherever along its course a sensory ner\e was 
stimulated, the resultant sensation was that appropri- 
ate to the sense organ it served. On the issue of elec- 
tricity in nerve, Miiller took the position that it was 
indeed an artificial excitant but had no part in natural 
excitation. He reached this conclusion largely from an 
experiment in which he mashed the nerve and demon- 

9 1 . Matteucci, C. Sur une phenomene physiologique produite 
par les muscles en contraction. Compt. rend. Acad, sc, 
Paris 4: 797, 1842. 

92. Matteucci, C. and F. H. A. Humboldt. Sur le courant 
electrique des muscles des animaux vivants ou recemment 
tues. Compt. rend. Acad, sc, Paris 16: 197, 1843. 

93. M.\TTEUCCi, C. Compt. rend. Acad, sc, Paris 52: 231, 1861 ; 
53: 503, 1861; 56: 760, 1863; 65: 131, 1867. 

94. MiJLLER, Johannes (1801-1858). Handbuch der Physiologic 
des Menschen. Coblentz: Holscher, vol. I, 1833; vol. II, 
1840; English translation by William Baly. vol. I, 1838; 
vol. II, [842. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



strated that, although electricity passed through the 
damaged zone, mechanical stimulation of the nerve 
above the injury provoked no twitch. 

During the era of intense concentration on electro- 
physiology in the Italian and German schools, labora- 
tories in other countries were developing a different 
approach. Among these was that of Claude Bernard 
(95), pupil of Magendie. Claude Bernard made use of 
curare as a blocking agent, interpreted by him as a 
nerve poison that spared the muscle. He found that 
in a curarized preparation the muscle would not 
twitch if he stimulated it directly and hence concluded 
that normally transmission could not be electrical 
either. In these experiments he used the ingenious 
little stimulator built from a Voltaic pile of alternate 
copper and zinc plates that is shown in figure 16. 
He did not recognize that his failure to evoke a con- 
traction by direct stimulation of the muscle was due 
to his 'pile' giving too feeble a current. 

Miiller was the last of the great physiologists to 
retain a trace of vitalism in his thinking. This he 
probably owed to his exposure as a student at Bonn 
to Natiirphilosophie and the influence of its leader, 
Schelling (96). Although more extensively indoc- 
trinated in this sterile philosophy than Oersted had 
been, Miiller was later able to free himself more ea,sily 
from its stultifying effects, and he eagerly encouraged 
the physical and chemical approaches to biological 
experiment. Not a trace of vitalism is found in his 
pupils. 

Towards the half-century a marked swing away 
from the metaphysics of Natiirphilosophie char- 
acterized neurophysiology, du Bois-Rcymond con- 
sidered himself (and with some right) to be the 
champion of this movement which strove to explain 
all physiology on chemical and physical grounds. 
And in fact, as we have .seen, it was the physicists of 
the period who were contributing most of the new 
experiments and concepts of muscle and peripheral 
nerve action. Before this, neurophvsiologists had 
reached a stage in their work in which progress was 
hampered by lack of sufficiently sensitive instruments. 
The physicists came to their help and indeed were 
themselves intrigued by the types of physical phe- 
nomena that biological preparations provided. 

In 1841 du Bois-Reymond received from his 

95. Bernard, Claude (1813-1878). Lf(,ons sur la phynoloiie 
el la pathologie du systeme nerveux. Paris: Bailli^re, 1B58. 2 vol. 

96. Schelling, Frederick Wilhelm Joseph (i 775-1854). 
Sammiliche Werke. .Stuttgart and Augsburg, 1856-1861, 
14 vol.; English translation of vol. Ill by T. Davidson. 
In J. Specula! . Philos. I: 193. 1867. 




% 



\ I 




FIG. 16. Claude Bernard at the age of 53, and the ingenious 
stimulators he used in his electrophysiological studies of nerve. 
They were miniature voltaic piles built up of alternate discs of 
copper and zinc. Just before use they were moistened with 
vinegar. Such dc\ices were made obsolete by the du Bois- 
Reymond induction coil and it is rather surprising to find 
Bernard still advocating them in his day. Although adequate 
for nerve stimulation, they gave too feeble a current to stimulate 
a muscle directly; from this Bernard concluded that the nervous 
effect on muscle could not be electrical. 



master a copy of Matteucci's book Essai sur les 
Phenomenes Electriques des Animaux (97), together with 
the suggestion that he repeat and extend Matteucci's 
experiments. By November of that year he had al- 
ready completed a preliminary note C98), but his 
major work, the Thierische Elektricitdt C99)j did not 
appear until 1848. The first part of this long and de- 
tailed book, unlike its later sections, shows little 
originality in scientific ideas, the author with a chip 
on his shoulder being carried along in the wake of 
Matteucci of whose publications he was outspokenly 
critical. However, where du Bois-Reymond shines, 
and what makes his book a classic, is his skill in in- 
struinentation, far surpassing that of Matteucci, so 
that he was able to extend and improve on these 
earlier observations. Moreover, not being hampered 
(as Matteucci was) by residual traces of a belief in 

97. Matteucci, C. Essai sur les Phenomenes electriques des 
Animaux. Paris; Czirilian-Goeury and Dalmont, 1840. 

98. DU Bois-Revmond, Emil (18 18-1896). Vorlaufiger Abriss, 
einer Untersuchung iiber den elektromotorischen Fische. 
Ann. Physik. Chem. 58: i, 1843. 

99. DU Bois-Revmond, E. Uniersuchungen iiber thierische Elek- 
tricitdt. Berlin: Reimer, vol. I, 1848; vol. II, 1849. 



22 



HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 





FIG. 17. du Bois-Reymond and one of the schemata he postu- 
lated for transmission at the end plate. 



'nerve force,' he brought clearer inductive rea.soning 
to the interpretation of his obser\'ations. 

du Bois-Reymond confirmed Matteucci's demon- 
stration that not only nerve-muscle preparations but 
muscles themselves could produce electricity and, 
with .soine acerbity, claimed priority for naming this 
the 'muscular current' (Muskelstrom). Both Mat- 
teucci and du Bois-Reymond distinguished muscular 
current from the frog current' (la correnta propria 
della rana), so named by Nobili to describe the current 
flow between the feet of the prepared frog and any 
other part of the animal. Neither Noijili (100) nor 
Matteucci, nor even du Bois-Reymond at this time, 
recognized that the so-called frog current was an 
injury current consequent to their having trans- 
sected their frogs. Nobili had thought it was a thermo- 
electric effect due to differential cooling times of 
nerve and muscle. 

du Bois-Reymond, using faradic stimulation, also 
confirmed Matteucci's finding that the muscle current 
was reduced during tetanic stimulation and named 
this the negative variation. It is what is now called 
the action current of muscle, du Bois-Reymond went 
on to demonstrate the same negative variation in 
nerve during activity and thus discovered the action 
current of nerve which Matteucci had failed to find 
with his less sensitive instruments, du Bois-Reymond 
made the following claim, "If I do not greatly de- 
ceive myself, I have succeeded in realizing in full ac- 
tuality (albeit under a slightly different aspect) the 
hundred years' dream of physicists and physiologists, 
to wit, the identity of the nervous principle with 
electricity." His great contemporary Carl Ludwig 
(loi) was unwilling to accept this for, thinking still 
in terms of the nerve as a telegraph wire, he held 

100. Nobili, L. Ann. chiin. el phys. 38: 225, 1828; 44: 60, 1830. 



(among other objections) that its resistance was too 
great and its insulation too poor for it to be a good 
conductor. 

Pfliiger (102) tried to o\ercomc some of these 
difficulties by his 'liberation hypothesis.' In this he 
stated that nervous transmission was "not a simple 
advancing undulation in which the sum of the living 
forces is not increased" but a situation in which "new 
tension forces are set free by the living forces of the 
stimulus and become in turn living forces with each 
onward step." In spite of the obscurity of the termi- 
nology (this is Morgan's translation), one can detect a 
foreshadowing of the ideas held by today's physi- 
ologists. 

du Bois-Reymond elaborated a theory that all un- 
damaged muscle had a resting potential between the 
middle (positive) and the tendons (negative) and 
that during activity this decreased, thus giving the 
'negative variation.' He was still not clear on the 
role of injury currents for he thought injury merely 
intensified the resting potentials. On this point he 
entered into acrimonious dispute with his pupil Her- 
mann who was equally stubborn in insisting that 
there were no resting potentials in the absence of in- 
jury and that all current flow in muscle and nerve 
was due to damage (103). Hermann therefore intro- 
duced the term 'demarcation currents' to describe 
them. Later experimentation has shown both men to 
have been partially right and partially wrong. 

du Bois-Reymond's conception of regularly oriented 
'electromotive particles' arranged along the surface 
of muscle and of nerve was the forerunner of the 
schemata of polarization that were to be developed 
more fully and more accurately by his pupil Berns- 
stein (104) and that lie at the core of modern theory. 
The critical issue as to whether the negative variation 
in nerve potential was identical with the excitatory 
process (i.e. the nerve impulse) was taken up by 
Bernstein who set out, at du Bois-Reymond's sugges- 
tion, to compare their velocities, von Helmholtz, one 
of the same brilliant group schooled in Miiller's 
famous laboratory, had in a triumph over primitive 
apparatus succeeded in measuring the velocity of 

101. Ludwig, C.-^rl (1816-1895). Uber die Ki-afte der Ner- 
venprimitivenrohr. Wien. med. Wchnschr. 46: 47, 1861. 

102. Pfluger, E. (1829-1910). Vnter suchungen iiber die Physi- 
ologie des Eleclrolonus. Berlin: Hirschwald, 1859. 

103. Hermann, Ludim.\r (i 838-1 91 4). W'eitere Untcrsungen 
iiber die Ursache der electro-motorischen Erscheinungen 
an Muskeln und Nerven. Arch. ges. Physiol. 3:15, 1870. 

104. Bernstein, Julius (1839-1 9 17). Untersuchungen iiber den 
Erregungsvorgang im Nerien- und Muskelsysleme. Heidelberg : 
Winter, 1871. 



THE HISTORICAL DEVELOPMENT OF NEUROPHVSIOLOGV 



23 



the excitatory processes (105) in the frog. In his 
success he had proved his old teacher wrong. In 1844 
Miiller had said, "The time in which a sensation 
passes from the exterior of the brain and spinal cord 
and thence back to the muscle so as to produce a con- 
traction, is infinitely small and immeasurable." von 
Helmholtz's technique was as follows: the moment of 
nerve stimulation, by the break shock of an induction 
coil, was signalled by the closing of the primary cir- 
cuit. The resultant muscle contraction lifted a contact 
in the same circuit, thus breaking it. The break sig- 
nalled the arrival of the nerve impulse in the muscle. 
By timing this inter\al, with stimulation at measured 
distances along the nerve, von Helmholtz was able 
to calculate its conduction velocity. This simplified 
description masks the extreme ingenuity of the original 
experiment. In technique von Helmholtz had coine 
a long way from Haller's attempt to discover the 
velocity of nervous action. Haller had read parts of 
The Aeneid a\oud, timing himself, counting the syllables 
and calculating the length of the nervous paths used 
in reading and speaking. In some way that is not 
entirely clear, he arrived at a figure of 50 m per sec. 

The conduction rate found by Bernstein (approxi- 
mately 29 m per sec.) tallied sufficiently well with 
von Helmholtz's final results, 27 to 30 m per sec, for 
him to be satisfied with the inferred identity of the 
impulse and the negative variation. Bernstein's experi- 
ments, using for stimulation a rheotome devised by 
himself with a galvanometer for detection of response, 
enabled him to plot the time course of what we now 
call the nerve's action potential and to determine its 
latency, rise-time and decay. One of the pregnant ob- 
servations he made was that the negative variation 
caused a deflection of his galvanometer that some- 
times crossed the base line, thus exceeding the value 
for the resting nerve potential. In today's terminology, 
he found the overshoot of the action potential beyond 
the resting potential level. 

Bernstein (106) became widely known for his 
theory that the membrane of the inactive fiber of 
nerve or muscle was normally polarized, having po.si- 
tive ions on the outside and negative ions on the in- 
side, and that the action potential was a self-propa- 
gating depolarization of this membrane. This was 

105. VON Helmholtz, H. (1821-1894). Messungen iiber den 
zcitlichen Verlauf der Zuchung animalischer Muskein 
und die Fortpflanzungsgeschwindigkeit der Reizung in 
den Nerven. Arch. Anat. Physiol. 111, 1850. 

106. Bernstein, J. Uber den zeitlichen Verlauf der negativen 
Schwankung des Nervenstroms. Arch. ges. Physiol, i : 1 73, 
1868. 



based on his assumption that the membrane is se- 
lectivelv permeable to potassium ions. His explana- 
tion of injury currents was that they were the result 
of a break in the membrane. 

In the later nineteenth century, after a long hiatus, 
phvsiology in England was again coming into its own. 
At the half-century, which saw such brilliance in the 
German schools, there was virtually no physiological 
work in progress in England. There were no physi- 
ological laboratories and there was no systematic 
physiological research. A dual chair in anatomy and 
physiology had i:)een created in 1836 at University 
College, London, and had been given to the anatomist 
William Sharpey. Such teaching as he gave in physi- 
ology was from books and his pupils saw no experi- 
ments, yet from among them came the leader of one 
of the more famous English schools of physiology, 
Michael Foster (1836-1907), founder of the Cam- 
bridge School. Though not himself a neurophysi- 
ologist, Foster could count among his pupils some to 
become later among the most brilliant in the field, 
Sherrington (1857-1952), Gaskell (1B47-1914), Lang- 
ley (1852-1925) and, as descendents from the last, 
Keith Lucas and in turn Adrian. 

This, the late nineteenth century, was an age of 
great progress in the development of instrumentation 
and, with their improved tools, physiologists were able 
to make more accurate observations of stimulus 
strength, response characteristics and time relation- 
ships than had their predecessors. In 1871 Bowditch 
(107) demonstrated that heart muscle did not respond 
with graded contractions to graded stimuli. He as- 
sumed that the global response he observed was due 
to a leakage of excitation throughout the fiber popu- 
lation of cardiac muscle. It was in fact the experi- 
mental evidence for what was later to be called the 
'all-or-nothing law.' Bowditch, an American, did 
these experiments in Ludwig's laboratory in Leipzig 
where he worked on the problem with Kronecker, the 
teacher of Harvey Cushing. On his return to Harvard, 
Bowditch founded the first laboratory for physiological 
research in the United States. 

Forgotten by Bowditch, or unread, were the writ- 
ings of Fontana in the eighteenth century in which, 
in discussing heart muscle, he said, "... the irritability 
of the fibre can be activated by a small cause, and by 
a feeble impression : but once activated, it has a 
power proportional to its own forces, which can be 

107. Bowditch, H. P. (1840-191 1). Uber die Eigenthumlich- 
keiten der Reizbarkeit welche die Muskelfasern des 
Herzens zeigen. Bcr. Konigl. Sachs. Gesellsch. Wiss. 23: 
652, 1 87 1. 



24 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



much greater than those of the exciting cause. . . ."'^ 
Fontana (loS) went on to a recognition of the re- 
fractory period (a term introduced by Marey) in 
heart muscle which he explained as an exhaustion 
of irritability resulting from the contraction. 

That skeletal muscle might share this property 
was also foreshadowed by Fontana but did not receive 
experimental proof until the work of Fick (109), an- 
other pupil of Ludwig's, although the finding was 
not further developed until the ingenious experi- 
ments of Keith Lucas (no) at the beginning of this 
century. In the meantime, an all-or-nothing property 
in nerve had been detected by Gotch (m), the 
predecessor of Sherrington in the Chair of Physiology 
in Liverpool, a finding that was to reach definitive 
form in the hands of Keith Lucas' pupil, Adrian (i 12, 
113). That the law applied to sensory as well as to 
motor nerves was established by Adrian & Forbes 
(114) in 1922 (in a paper whose title replaced the 
term 'all-or-none' by the more grammatical one 
'all-or-nothing'). This line of work led on to investi- 
gations of the refractory period of peripheral nerve 
and the accurate plotting of the time course of after 
potentials. The invention of the vacuum tube ampli- 
fier and the cathode ray oscillo.scope opened the 
modern era of electrophysiology, and with them the 
foundations of today's techniques were laid by 
Gasser & Erlanger (i 15). 

One branch of peripheral nerve physiology remains 

108. Fontana, Felice Caspar Ferdinand (1730-1805). De 
Legibus Irritabilitatis. Lucca : Riccomini, 1 767. 

109. Fick, Adolf (1829- 1901). Mechanische Arbeit und Wurme- 
fntivicklung bei der Miiskelthritigkeit. Leipzig: Brockhaus, 
1882. 

1 10. Lucas, K. The "all-or-nonc" contraction of ampiiibian 
skeletal muscle. J. Physiol. 38: 113, igog. 

111. Gotch, Francis (i853-igi3). The sub-maximal electrical 
response of nerve to a single stimulus. J. Phyuol. 28: 

395. 1902- 

112. Adrian, Edgar D. (i88g- ). On the conduction of 
subnormal disturbances in normal nerve. J. Physiol. 43: 
389, 1912. 

113. Adrian, E. D. The "all-or-none" principle in nerve. J. 
Physiol. 47: 460, 19 1 4. 

114. .Adrian, E. D. and A. Forbes. All-or-nothing responses 
in sensory nerve fibres. J. Physiol. 56: 301, 1922. 

115. Gasser, Herbert S. (1888- ) and Joseph Erlanger 
(1874- ). A study of the action currents of nerve 
with a cathode ray oscillograph. Am. J. Physiol. 62: 496, 
1922. 

" Quoted from Hoff, H. E. The history of the refractory 
period. Yale J. Biol. & Med. 14: 635, 1942. 



to be outlined. This is the subject of neuromu.>;cular 
transmission. Its history is short for, before the latter 
half of the nineteenth century, continuity between 
nerve and muscle was assumed, the neuron theory 
had not been formulated and neuroneural synapsis 
had not been conceived. The i 700-year-old hypothe- 
sis of a nervous fluid implied humoral transmission in 
structures having continuity and only at mid-nine- 
teenth century, when this was finally abandoned, did 
the possibility of junctional tissues become a live one. 

In 1862 Willy Kiihne (116, 117), pupil of von 
Briicke and later professor of physiology in Heidelberg, 
published a memoir on the end organs of motor nerves. 
Noting the histological differences between muscle 
and its innervating ner\e, he suggested that action 
currents of the nerve by invasion of the muscle 
caused it to contract. That there was a delay at the 
neuromuscular junction was noted in du Bois-Rey- 
mond's laboratory and the master him.self considered 
the possibility of a chemical influence (the agents he 
mentioned were ammonia and lactic acid which 
Leibig had demonstrated in muscle in 1847); he went 
to great pains, however, to sketch electrical fields in 
support of what was called the 'modified discharge 
hypothesis' (as shown in fig. i 7). 

The controversy surrounding the mode of trans- 
mission at the motor end plate was carried into the 
modern era and, at a time not yet history, essential 
agreement was reached that transmission at the neuro- 
muscular junction is chemical in nature. The major 
contribution that settled the issue came from pharma- 
cological experimentation of today's scientists, stem- 
ming from the pioneer work of Elliott (118), Dale 
(119) and Loewi (120) in the early part of the cen- 
tury. Elliott, while a student at Cambridge, noticed 
that smooth muscle responded to adrenin even when 
deprived of its sympathetic nerves and this led him 

116. KiJHNE, Willy (1837- 1900). Uber die periphniuheii 
Endorgane der molorisehrn .Herven. Leipzig: Engelmann, 
1862. 

117. Kuhne, VV. On the oiigin and causation of vital move- 
ment. Proc. Roy. Soc, London, ser. B 44: 427, 1888. 

118. Elliott, Thomas Renton (1877- ). On the action 
of adrenaline. J. Physiol. 32: 401, 1905. 

119. Dale, Henry Hallett (1875- ). Transmission of 
nervous effects of acetylcholine. Harvey Lectures 32: 229, 

'937- 

120. LoEVvi, Otto (1873- ). Uber humorale Ubertrag- 
barkeit der Hcrtznervenwirkung. Arch. ges. Physinl. iBg- 
23g, 1 92 1. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



to suggest that adrcnin "might then be the chemical 
stimulant liberated on each occasion when the im- 
pulse arrives at the periphery." Langley (121), who 
was at that time professor of ph\ siology at Cambridge, 
recognizing that in some smooth muscle the action 
both of sympathetic nerve stimulation and of adrenin 
was to produce contraction whereas in others the 
result was a relaxation, postulated the existence of 
two kinds of receptor substance — excitatory and in- 
hibitory. That adrenin mimicked sympathetic action 
was then accepted. 

The possibility of a chemical mediator for the 
vagal action on the heart was explored experi- 
mentally in several centers. Bottazzi (122), Martin 
(123) and Howell (124) thought the agent must be 
potassium, Dixon (125) that it was muscarine, an 
alkaloid closely related in structure to the cholines. 
These substances had been shown to be active in sev- 
eral puzzling ways. In 1906 Hunt & Taveau (126) 
had demonstrated the extremely potent effect of 
acetylcholine on arterial pressure, and by 1914 the 
work of Dale (127) was already pointing so strongly to 
acetylcholine being the drug involved in parasympa- 
thetic action, that he described it as 'parasympatho- 
mimetic' Direct experimental proof was lacking that 
a chemical substance excreted as a result of nerve 
stimulation would in fact activate a tissue in a similar 
way, although the hypotheses both for epinephrine in 
the sympathetic and acetvlcholine in the parasympa- 
thetic system seemed highly plausible. 

The direct proof came from the brilliant researches 
of Otto Loewi (128) in which he demonstrated that 

12!. Langley, John Newport (1852-1906). On the reaction 
of cells and of nerve-endings to certain poisons, chiefly as 
regards the reaction of striated muscles to nicotine and to 
curare. J. Physiol. 33: 374, 1905. 
Bottazzi, P. Arch. Physiol. 882, 1896. 
Martin, E. G. The inhibitory influence of potassium 
chloride on tlie heart, and the eff^ect of variations of 
temperature upon this inhibition and upon vagus in- 
hibition, .-im. J. Physiol. II : 370, 1904. 
Howell, VV. H. Vagus inhibition of the heart in its re- 
lation to the inorganic salts of the blood. Am. J. Physiol. 
15: 280, 1906. 

Dixon, W. E. On the mode of action of drugs. Med. Mag. 
16:454, '907- 

ijfi. Hunt, R. and R. de M. Taveau. On the physiological 
action of certain cholin derivatives and new methods for 
detecting cholin. Bril. A/. J. ■2: 1788, 1906. 

127. Dale, H. H. The action of certain esters and ethers of 
choline, and their relation to muscarine. J. Pharmacol. 
& Exper. Therap. 6: 147, 1914. 

128. Loewi, O. Uber humorale Ubertragbarkeit Herz- 
nervenwirkung. .^rch. ges. Physiol. 189: 239, 192 1. 



122 
123 



124. 



125- 




^^ 



FIG. 18. Lift: an early representation of spinal roots and 
tracts as drawn by Domenico Mistichelli in his Traltalo dtU'.i/io- 
plessia, 1 709 (from the copy in the Boston Medical Library by 
courtesy of Dr. Henry Viets). Mistichelli is considered to be 
one of the first workers to recognize the crossing of the pyra- 
mids. Right: the crossing of the pyramids was described and 
experimentally demonstrated on injury to the brain in dogs by 
Pourfour du Petit, a pupil of Duverney. His drawings are from 
his Lettres d'un medicin, 1 727. (From the copy in the Bibliotheque 
Nationale. Reproduction by courtesy of Dr. Auguste Tournay.) 



the fluid bathing a frog's heart which had been stimu- 
lated through its vagus had an inhibitory action on 
the beat of another heart. He named the agent 
'Vagusstoffe.' From this cla.ssic observation, one of 
the landmarks of physiology, experimentation spread 
cut to the examination of other tissues, other nerves, 
and other mediators and inhibitors, and forms one of 
the wide fields of today's research. With the recogni- 
tion of neuroneural synapses the problem of trans- 
mission was carried from the peripheral neuromuscu- 
lar svstem into the central nervous svstem. 



SPINAL CORD AND REFLEX ACTIVITY 

The functions of the spinal cord long remained an 
enigma to the early physiologists. For as long as the 
belief persisted that every nerve in the body required 
its own canal leading directly from the brain in order 
to insure its supply of animal spirits, the spinal cord 
appeared to be merely a bundle of nerve fibers 
grouped together. In other words, it was a prolonga- 
tion of the peripheral nervous system channeling into 
the brain. 



26 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



The relationship of the spinal cord to peripheral 
nerves and to the rest of the central nervous system 
could hardlv be understood until the structure of the 
neuron had been learned. The period that saw the 
great development of knowledge of cell structure came 
with the high-power microscopes of the nineteenth 
century. Before then descriptions of the finer elements 
necessarily lacked exactness, though in 1767 Fontana 
had given a good account'^ of the axis cylinder, and 
there seems little reason to doubt that the bodies 
Alexander Monro (129) saw in the spinal cord in 
1783 were the anterior horn cells. Nerve cells were 
certainly seen by Dutrochet (130) in 1824 though we 
do not find a very exact description of them before 
1833, when Ehrenberg (131, 132) published his find- 
ings on the spinal ganglia of the frog. 

The visualization of axis cylinders on the one hand, 
and of cell bodies on the other, still did not help the 
physiologist very much in his search for understanding 
of nervous connections. It was from the botanists that 
the next lead came. The cell theory had a long history 
among plant physiologists and its emphasis on the 
role of the nucleus and the cellular matrix appealed 
to microscopists who could see similar structures in 
animal tissues. In 1837 Purkinje (133), working at 
home for lack of a laboratory at the Universit>- of 
Breslau where he was profes.sor, realized the .signifi- 
cance of the ob.servations on plant tissues and sug- 
gested that the cell theory might justifiably be ex- 

129. Monro, .\lex.\nder (secundus) (1733-1817). Observa- 
tions on the structure and Junctions of the nervous system. 
Edinburgh: Creech, 1783. 

130. Dutrochet, Rene Joachim Henri (i 776-1847). ^f- 
ckerches anatomiques et physiolooiques sur la structure intime 
des animaux et des vegetaux. Paris : Bailliere, 1 824. 

131. Ehrenberg, C. G. Notwendigkeit einer feineren mecha- 
nischen Zerlegung des Gehirns und der Nerven. Ann. 
Physik. u. Chem. 104: 449, 1833. 

132. Ehrenberg, C. G. Beobachtung einer unhekannten Structur 
des Seelesorgans. BerHn, 1836. 

133. Purkinje, Johann Evangelista (1787-1869). Uber die 
gangliose Natur bestimmter Hirntheile. Ber. Versamml. 
deutsch. Natmjorsch. Artze, Prague 1837, p. 175. 

'^ "Le nerf est forme dun grand nombre de cylindres 
transparents, homogenes, uniformes, tres-simples. Ces cylindres 
paroissent formes, comme d'une parol, ou tunique tr& subtile, 
uniforme, remplie, autant I'oeil peut enjuger, dune humeur 
transparente, gelatineuse, insoluble dans I'eau. Chacun de ces 
cylindres recoil une enveloppe en forme de gaine exterieure, 
la quelle est composee d'un nombre immense de fils torteux." 
Fontana. Traite sur le venin de la I'ipere. Florence, 1781.2 vol. 



tended from botany to zoology. Two years later 
Schwann (134) marshalled the facts and crystallized 
the idea in his classic monograph. 

For an understanding of function, knowledge of the 
cell bodies was not enough. The nerve tracts were of 
primary importance, and during this same period 
histologists were finding that the medullated axon was 
not the only kind of fiber. In 1838, in a little book that 
was one of the last scientific texts to be published in 
Latin, Remak (135) revealed the existence of non 
medullated nerves. His work is illustrated by many 
delicate drawings of cells from various parts of the 
nervous system, mostly taken from ox and man. But 
by 1865 phy.siologists knew that in addition to medul- 
lated and nonmedullated nerves there were other 
fibrous processes which Dieter's (136) work (published 
posthumously) showed to be dendrites. In the saine 
monograph there is a description of the glia. The cell 
theory did not explain how all these fibrous structures 
related to the cell body, and a student's thesis was one 
of the early publications to take this step. In 1842 
von Helmholtz (137), in the earliest of the many 
brilliant contributions he made to physiology, estab- 
lished the connection between peripheral nerve and 
ganglia in invertebrates using the crab, von Helmholtz 
was 2 1 years old when he wrote this inaugural thesis. 

The next major advance came in 1850 from Waller 
(138) with his demonstration that axons degenerate 
when cut off from their cell bodies and his conclusion 
that the latter were their source of nutriment. The 
development by Marchi & Algeri (139) of the osmic 
acid stain for degenerating myelin sheaths gave the 
physiologist a technique for tracing the nerve tracts. 

134. Schwann, Theodore (1810-1882). Mikroskopische Unter- 
suchungen iiber die Ubereinstimmung in der Struktur und dem 
Wachsthum der Thiere und Pflanzen. Berlin: Sander, 1839; 
English translation by Sydenham Society, 1847. 

135. Rem.\k, Robert (181 5-1 865). Observationes anatomicae el 
microscopuae de systematis nervosi structura. Berlin: Reimer, 
1838. 

136. Dieters, Otto Friedrich Karl (1821-1863). Unter- 
suchungen iiber Gehirn und Riickenmark des Menschen und der 
Sdugetiere. Brunswick: Vieweg, 1863. 

137. VON Helmholtz, H. De Fabrica systematis nervosi Everte- 
bratorum (Inaugural Thesis). 1842. 

138. Waller, .Augustus Volney (1816-1870). Experiments 
on the section of the glossopharyngeal and hypoglossal 
nerves of the frog, and observations of the alterations 
produced thereby in the structure of their primitive 
fibres. Phil. Trans. 140: 432, 1850. 

139. Marchi, V. and C. Algeri. Sulle degenerazioni discen- 
denti consecutive a lesioni della corteccia cerebrale. Riv. 
sper. Frernat H: 492, 1885. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



27 




TAB 11; 1 



FIG. iq. Left: Jiri Pfochaska of Prague, the proponent of 
automatic reflexion in the medulla and spinal cord. Ri«ht: 
Prochaska's illustration of the spinal roots and their ganglia. 

The definitive study of the relationship of the medul- 
lated axon to the nerve cell followed in 1889 and was 
the work of von Kolliker (140), profes.sor of anatomy 
in Wiirzburg. From this wealth of accumulated 
knowledge, a generalized concept of neuron behavior 
became possible and in i8gi a clear formulation was 
achieved by Waldeyer-Hartz (141). The neuron 
theory was established. In reviewing thfse basic steps 
that had to be taken before any unravelling of central 
nervous system pathways could proceed with cer- 
tainty, one is struck by the fact that so many of the 
contributors (Schwann, Remak, von Helinholtz, 
Kolliker) were pupils of Johannes Miiller. 

Another of the early stumbling blocks to an under- 
standing of the spinal cord was the difTerentiation of 
motor and sensory function. It was early suspected 
that the ganglia of the spinal roots were in some way 
involved in this question. Galen had thought that the 
presence of a ganglion indicated that the nerve was 
powerfully motor in action and here the matter rested 
for some centuries. In 1783 Alexander Monro (129) 
noted that the spinal ganglia were formed on the 

140. VON Kolliker, Rudolf Albert (1817-1905). Mikro- 

skopische Analomie. Leipzig, 1850- 1854. 
14L Waldever-Hartz, Heinrich Wilhelm Gottfried 

(1836- 1 921). Uber einige neuere Forschungen im Gebiete 

der Anatomic des Centralnervensystems. Deutsche med. 

Wchnschr. 17: 12 13, 1244, 1287, 1331, 1352, 189L 




posterior roots and that their coalescence with the 
anterior roots occurred peripherally to these swellings. 
But like Galen he thought that they were concerned 
with 'muscular' nerves and defended them as such 
against the suggestion by James Johnstone (142) that 
their action was to cut ofT sensation. This rather 
bizarre concept had received .some consideration in 
the mid-eighteenth century. 

The presence of ganglia suggested to several minds 
a specialization of function in the nerves on which 
they were formed. Both Prochaska C'43) 3"^ 
Soemmering (144) had drawn attention to the re- 

142. Johnstone, James (i 730-1802). Essay on the use of the 
ganglions of the nerves. Phil. Trans. 54: 177, 1765. 

143. Prochaska, Jiri (1749-1820). De Structura .Nervorum. 
Prague: Gerle, 1780-1784. 3 vol. 

144. Soemmering, Samuel Thomas (1755-1830). De bast 
encephali et originibus nervorum cranio egredienlum. Got- 
tingen : Vandenhoeck, 1778. 



28 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



semblance between the ganglia of the fifth cranial 
nerves and those of the posterior roots, and Bichat 
(145), the brilliant French pathologist who died so 
young, had gone so far as to associate all ganglia with 
the nervous processes of involuntary, unconscious 
'organic' life. 

The differentiation between the ganglia found in 
the sympathetic nersous system and those on the 
roots of the central nervous system was to come later. 
Charles Bell made the distinction but admitted he did 
not know what role was pla\ed b\- the .sympathetic 
nerves or by their ganglia (146). His many studies on 
the fifth and seventh cranial nerves (146-148), illus- 
trated by his own beautiful drawings, are classics, and 
his demonstrations of the function in the nerves of 
the face are perpetuated in the name Bell's palsy. 
Bell had come from Edinburgh to the famous ana- 
tomical school that William Hunter had founded in 
Great Windmill Street near Piccadilh'. A brilliant 
di.ssector but not primarily an experimentalist. Bell 
relied heavily on his brother-in-law, John Shaw, in 
this aspect of his work and suffered a great loss when 
Shaw died. 

In the cord the various columns had been dissected 
by the anatomists and the grouping together of nerves 
in such larue bundles had certainly seemed suggestive 
of parcellation of function. But not all anatomists 
were agreed. Bichat on dissecting out some nerve 
filaments found them centrally located in the lower 
cord but more lateral higher up. He therefore con- 
cluded that although the filaments had individual 
properties, the fasciculi were mixed. The idea per- 
sisted, however, that the columns and also the spinal 
roots might have different functions according to 
whether they were anterior or posterior. An early idea 
was that the anterior roots carried Ijoth motor and 
sensory supplies for the muscles while the posterior 
roots gave a sensory service for the skin. An Edinburgh 

145. Bichat, Marie Francois Xavier (1771-1802). Aualomie 
generale, appliquee a la physiologie el a la medecine. Paris: 
Brosson, 1801, 2 vol., English translation by G. Hay ward 
Boston: Richardson and Lord, 1822. 3 vol. 

146. Bell, Charles (1774-1842). The Nervous System of the 
Human Body as explained in a series of papers read be/ore 
the Royal Society 0] London. Edinburgh: Black, 1836. 

147. Bell, C. On the nerves; giving an account of some ex- 
periments on their structure and functions, which lead 
to a new arrangement of the system. Phil. Trans. 1 1 1 : 398, 
1 82 1 . 

148. Bell, C. Of the nerves which as.sociatc the muscles of the 
chest in the actions of breathing, .speaking, and expres- 
sion. Being a continuation of the paper on the structure 
and functions of the nerves. Phil. Trans. 112: 284, 1822. 



anatomist, Alexander Walker Ci49)> suspected that 
they might serve .separate roles but unfortunately 
picked the posterior root as the motor and the 
anterior root as sensory. 

In 181 I a small pamphlet was pri\-ately primed, 
entitled Idea oj o new anatomy of the brain suhmitled jir 
the observation of his friends. The author was Charles 
Bell (150). This pamphlet had no general distribution, 
no more than 100 copies being printed. (Only three 
are known to exist today, one of which is in the Na- 
tional Library of Medicine in Washington; in Eng- 
land, copies can be seen at the British Museum and 
at the Royal Society.) Bell stated that the purpose of 
this pamphlet was to assure his friends that in his dis- 
sections of the brain he was investigating its structure 
and not searching for the seat of the soul. In this work 
he stated his opinion that nerves owe their differences 
in properties to their being connected to different 
parts of the brain. He said that, holding this opinion, 
he wondered whether the double roots of the spinal 
nerves might indicate that "nerves of different en- 
dowments were in the same cord, and held together 
by the same sheath." To test this idea experimentally, 
he cut "across the posterior fasciculus" and noted 
that there were no convulsive movements of the 
muscles of the back; but that on touching the anterior 
fa.sciculus with the point of a knife, the muscles of the 
back were immediately convulsed. From this experi- 
ment he concluded at that time, "The spinal nerves 
being double, and having their roots in the spinal 
marrow, of which a portion comes from the cerebrum 
and a portion from the cerebellum, they convey the 
attributes of both grand divisions of the brain to every 
part, and therefore the distribution of such nerves is 
simple, one nerve supplying its distinct part." 

It may be noted that there is in this pamphlet no 
suggestion that the posterior columns or roots might 
be sensory in function. Bell considered the cerebellum 
to be concerned with involuntary and unconscious 
functions ("the .secret operation of the bodily frame" 
and "the operation of the viscera") whereas he recog- 
nized the cerebrum "as the grand organ by which 
the mind is united with the body. Into it all the nerves 
from the external organs of the senses enter; and from 
it all the nerves which are agents of the will pass out." 

149. Walker, Alexander (1779-1852). New anatomy and 
physiology of the brain in particular, and of the nervous 
system in general, -irch. Universal Sc. 3: 172, 1809 

1 fjo. Bell, C. Idea of a new anatomy of the brain submitted for the 
observation of his friends. Privately printed, 1811; repro- 
duced in J. F. Fulton. Selected Readings in the History of 
Physiology. Springfield: Thomas, 1930, p. 251. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



29 



In essence, thcrcroic, Bell regarded the cerebellum, 
posterior columns and posterior spinal roots as con- 
cerned with unconscious impressions and involuntary 
movements; the cerebrum, anterior columns and 
anterior roots as conveying conscious sensation and 
willed moxements. 

On Julv 22, 1822, Frangois Magendie, member of 
the Academy of Sciences of Paris (and later to be 
professor at the College de France), read a paper (151) 
to the Academy as a result of which the following 
entry was made: "M. Magendie reports the discovery 
he has recently made, that if the posterior roots of the 
spinal nerves are cut, only the sensation of those nerves 
is abolished, and if the anterior roots arc cut, only the 
movements they cause are lost." This report was 
followed by a fuller account (152, 153) in the journal 
that Magendie himself had founded. The experiments, 
made on puppies which survi\ed the surgical pro- 
cedures, gave Magendie the confidence to state "that 
the anterior and posterior roots of the nerves which 
arise from the spinal marrow, have different func- 
tions, that the posterior appear more particularly 
destined to sensibility, whilst the anterior seem more 
especially allied to motion." 

In spite of his not having suggested a function of 
conscious sensation for the posterior roots in either 
the privately printed pamphlet or published papers 
(147) on the fifth and seventh cranial nerves, Bell 
with a questionable lack of scruple claimed full pri- 
ority and engaged in a wrangle that invaded the scien- 
tific journals for many years. This carried the un- 
pleasant flavor of evidence twisted by hindsight. Bell's 
"republications' in 1824 (154) of his earlier writings 
contained .subtle changes in wording that deceived 
his supporters into believing his claims to be better 
founded than they were.^^ Among those hoodwinked 
were Flourens and, at first, Magendie's pupil, Claude 
Bernard. Posterity gives each some credit by pre- 

151. Magendie, Franqols (1783-1855). Proces-verb, 1822. 
Acad. Sc. 7: 348, 1820-1823. 

152. Magendie, F. Experiences sur les fonctions des racines 
des nerfs rachidicns. J. phystol. exper. et path. 2; 276, 1822. 

153. Magendie, F. Experiences sur les fonctions des racines 
des nerfs qui naissent de la moelle epiniere. J. physiol. 
exper. et path. 2: 366, 1822. [References 152 and 153 can 
be read in English in Alexander Walker's translations 
in. Documents and dales of modern discoveries in the nerv- 
ous system (Pub. anonymously) London: Churchill, 

1 839-] 

154. Bell, C. An Exposition of the Natural System of the Nerves of 
the Human Body with a Republication of the Papers Delivered 
to the Royal Society, on the Subject of .Verves. London: .Spot- 
tiswoode, 1824. 




FIG. 20. The protagonists in the Bell-Magendie controversy. 
Bell (/f//) and Magendie Qright} as young men. The portrait 
of Bell was painted by .Antony Stewart of Edinburgh in 1804; 
that of Magendie (attributed to Guerin) is at the College de 
France. 



serving the nomenclature of the Bell-Magendie Law. 
In spite of his claims. Bell made no move to get ex- 
perimental proof of the function of the posterior roots 
and as late as 1832 (155) was stressing that their 
sensory nature was only inferred. He said in his lec- 
tures to the Royal College of Physicians, ". . .as we 
have proved the anterior column to be the origin of 
the motor nerves, we may infer the posterior roots are 
those which render the entire nerve a nerve of sensa- 
tion." In 1844 Johannes Miiller (156) confirmed the 
law experimentally, something Bell had never done, 
but the conclusion seems inescapable that the concept 
in its complete form as well as its experimental proof 
was first contributed by Magendie. 

Magendie, whose youth coincided with the French 
Re\olution, came from surgery into physiology where 
his urge towards experimentation could give him 
greater satisfaction. So strongly empiricist was he that 
he rarely made generalizations from his observations 

155. Bell, C. Lectures on the physiology of the brain and 
nervous system. Reported in: Ryan's Med. .Surg. J. i: 
682, 752, 1832. 

156. MtJLLER, J. Bestutigung des Bell'schen Lehrsatzes. Notiz. 
a. d. Geb. d. natur- u. heilk. (Weimar) 30: 113, 1831; 
this is more readily available in French in Ann. Sc. Natur. 
23: 95, 1831, and a section is translated into English in 
\V. Stirling. Some Apostles of Physiology. London : Waterlow, 
1902. 



'^ For a detailed comparison of the texts see Flint, A. Con- 
siderations historiques sur les proprietes des racines des nerfs 
rachidiens. J. de ianat. et de physiol. 5: 520, 1868. 



30 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



in the laboratory which were many and varied. His 
work on the spinal roots led him to follow the differ- 
entiation of function into the spinal tracts where he 
found that pressure on the posterior columns, but not 
on the anterior, caused signs of pain. One other aspect 
of Magendie's work on the spinal cord should be 
mentioned, his rediscovery of the cerebrospinal fluid 
(157). Sixty years earlier this had been seen and 
described by Cotugno (158), at that time a young 
physician in the Hospital for Incurables in Naples, 
but his monograph had stirred no general interest 
though it helped to win him the chair of anatomy at 
the university. Magendie described the forainen 
known by his name, but oddly revived a valve-like 
role for the pineal as controller of this opening. He 
thought the fluid was secreted by the arachnoid mem- 
brane, and it was many years later that its origin in 
the choroid plexuses was discovered. A later pamphlet 
by Magendie (159) on the cerebrospinal fluid has 
some fine illustrations by H. Jacob. 

Once the differentiation of function between the 
anterior and posterior roots had been accepted, the 
finer points as to which regions were inner\ated by 
their fibers began to occupy the physiologists. The 
question as to whether all the fibers of an anterior 
root served the same or many muscles was paralleled 
by its corollary as to whether one muscle received 
fibers from one or many roots. That the last arrange- 
ment is the correct one was first clearly shown by 
Eckhardt (160) in frogs and by Peyer (161) in rabbits. 
Both were working in Carl Ludwig's laboratory. The 
definitive demonstrations came later from Sherring- 
ton's (162) careful analyses, mostly in the monkey, 
from which he concluded that '"the position of the 

157. Magendie. Memoire sur la liquide qui se trouve 
dans le crane et canal vertebral de Thomme et des 
animaux mammifiercs. J. physiut. cxph. el path. 5: 27, 
1825. 

158. Cotugno, Domenico (i 736-1822). De Ischiade Nervosa 
Commentarius. Naples: Simonios, 1764; a portion has been 
translated into English. A Treatise on the Nervous Sciatica^ 
or Nervous Hip Gout. London: Wilkie, 1775, p. 14. 

159. Magendie, F. Rkherches physiologiques et cliniques sur le 
liquide cephalorachidien ou cerebro-spinal. Paris: Mequignon- 
Marvis, 1842. 

160. Eckhardt, C. Uber Reflexbcwcgungender vier letzten 
Nervenpaare des Frosches. ^Ischr. rat. Med. (ist series) 
I : 281, 1849. 

161. Peyer, J. Uber die pcriphcrischcn Endigungen der 
motorischen und sensibelen Fasern der in den Plexus 
brachialis des Kaninchens eintretenden Nerven wurzeln. 
Zlschr. rat. Med. (ist series) 4: 67, 1853. 

162. Sherrington, Charles Scott (1857- 1952). Notes on 
the arrangement of some motor fibres in the lumbosacral 
plexus. J. Physiol. 13: 621, 1892. 



nerve-cells sending motor fibres to any one skeletal 
muscle is a scattered one, extending throughout the 
whole length of the spinal segments innervating that 
muscle." 

Tracing of the fibers of the sensory roots was in- 
trinsically more difficult. Tiirck's (163) studies in 
X'ienna had indicated the complexity of sensory inner- 
vation in the dog, and Herringham (164) had found 
the segmental relationship with the vertebrae; but 
again it was Sherrington (165) who, using the reflex 
as criterion of the existence of afferent fibers, un- 
ravelled the phenomena of overlapping of segmental 
cutaneous innervation. Until the time of Sherrington 
it had been thought that the motor fibers to a given 
muscle were derived from the same spinal segment 
that received the sensory inflow from the skin sur- 
rounding it. This was particularly the view of Krause 
(166). Sherrington's mapping of myotomes and der- 
matomes showed this rule to be erroneous. 

.Sherrington's development of a comprehensive 
theory of reflex action could scarcely have been en- 
visaged before the sensory endings in muscle had been 
discovered. This advance was mainly the work of 
Rufiini (167, 168) who in 1892 identified as sensory 
organs muscle spindles, tendon organs and Pacinian 
(169) corpuscles. These structures had been seen and 
dcscriljed by others, but their function had not been 
appreciated. The need for an apparatus for muscle 
sense had been felt by Charles Bell (170) in order to 
convey "a sense of the condition of the muscles to the 
brain," and he postulated "a circle of nerves," saying 
that "every muscle has two nei^es, of different proper- 
ties supplied to it." That sensations are aroused by 

163. TiJRCK, Ludwig (1810-1868). Uber die Haut-Sensibili- 
tatsbewirke der enzelnen Riickenmarksnervenpaare. 
Denkschr. Akad. Wiss. 29: 299, 1868. 

164. Herringham, VV. P. The minute anatomy of the brachial 
plexus. Proc. Roy. Soc, London, ser. B 41: 423, 1887. 

165. Sherrington, C. S. Experiments in examination of the 
peripheral distribution of the fibres of the posterior roots 
of some spinal nerves. Phil. Trans. 184 B; 641, 1894. 

166. Krause, Fedor (1856-1937). Beitrdge zur Neurologie der 
oberen Extremildt. Leipzig, 1865. 

167. RuFFiNi, Angelo. Di una particolare reticella nervosa e 
di alcuni corpuscoli del Pacini che si trovano in conces- 
sione cogli organi musculo tendinei del gatto. Atti R. 
Accad. Lincei 1 : 12, 1889; French translation in: Sur un 
reticule nervcux special et sur quelques corpuscles de 
Pacini qui se trouvent en connexion avec les organes 
musculo-tendineux du chat. Arch. ital. biol. 18: loi, 1893. 

168. RuFFlNi, .\. Observations on sensory nerve endings in 
voluntary muscles. Brain 20: 368, 1897. 

169. Pacini, Filipfo (181 2-1883). -^uovi organi scoperti net 
corpo humani. Pistoja: Cino, 1840. 

170. Bell, C. On the nervous circle which connects the volun- 
tary muscles with the brain. Phil. Tram. 2: 172, 1826. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



31 



movements of the limbs is an oljservation that goes 
back at least to Descartes' posthumous treatise (171), 
but that the act of volition in itself could also be 'felt' 
was an idea espoused by some, including, rather sur- 
prisingly, von Helmholtz." But a peripheral rather 
than a central mechanism had more adherents for, 
like Bichat, they thought that muscles must be sensi- 
tive. 

Infiltrating the early work on spinal cord physi- 
ology is the gradual development of the idea of the 
reflex. The eventual emergence of a concept of reflex 
activity grew out of centuries of attempts to explain 
animal movements, motion receiving more attention 
than sensation for it was considered to be the sign of 
life. Galen had regarded movements as three in kind : 
natural (such as the pulse), governed by the heart; 
voluntary, governed by the soul (located in the brain); 
and unconscious movements of voluntary muscles 
(such as in respiration). Involuntary muscle was un- 
known even in the days of Fernel (i 72) and Descartes 
(i 73), both of whom emphasized a distinction between 
movements dictated by reason and those due to the 
appetites. The ideas of Fernel and of Descartes have 
both long been regarded as forerunners of the concept 
of reflex activity. The claims for Fernel rest on his 
observation of automatic movements, some of which 
we now know to be reflexly initiated; but the pe- 
ripheral origin or the stimulus that caused them was 
not recognized by him. An ardent supporter of 
Descartes as the originator was du Bois-Reymond 
(174) who stressed this claim in his eulogy of Miiller, 
written at the time of the latter's death. 

The first suggestion that perhaps the spinal cord 
could be a center for communication between nerves 
was made by Thomas Willis (i 75) who came very 
close to picturing the reflex. He thought that all 
voluntary movements came from the cerebrum, all 
involuntary from the cerebellum and that they were 
ruled by a soul that resided both in the blood and in 
the nervous fluid. For Willis the medulla was an 
appendix of the brain which he likened to a musical 
organ (30) taking air into its bellows (i.e. animal 
spirits from the brain) in order to blow them out into 

171. Descartes, R. Traite de I' Homme, first French ed. 1664, 
chapt. 77. 

172. Fernel, J. De Naiurali Parte Medicinae (ist ed.). Paris: 
Simon de Colines, 1542; 2nd ed. Physiologia. 1554. 

173. Descartes, R. Traite de I' Homme, first French cd. 1664. 

174. DU Bois-Reymond, E. Gedachnissrede auf Johannes Miiller. 
Berlin, 1858; reprinted in Reden, vol. 2. Leipzig: Veit, 
1887. 

175. Willis, Thomas (i 621 -1675). De Anima Brutorum {De 
Scientia seu Cognitione brutorum^. London: Davis, 1672. 



the appropriate organ pipes (the nerves). Elsewhere 
(176) Willis showed his interest in the organ as a 
musical instrument and gave some description of it. 

Where Willis came close to describing reflex action 
was in stating that sen.se impres.sions carried by the 
animal spirits to the sensorium commune (which he 
put in the corpus striatum) went on to higher levels 
of the cerebrum where they were perceived and 
formed into memories. Some, however, were reflected 
back towards the muscles ('species alia reflexa'). Al- 
though the resultant movement was automatic and 
although one might be unaware of the sensory stimu- 
lus, Willis held that one was conscious of the resultant 
muscular effect. The example he gives is irritation of 
the stomach causing vomiting, and it is noticeable 
that Willis's discussion of 'reflexes' comes in his chap- 
ter on knowledge and recognition. 

Willis used 'motus reflexus' and the verb refluere' 
in making this proposition and the terms were used 
again by Baglivi (i 77) who refers to him. Their usage 
of 'reflexus' reads as though it were closer to the 
modern term than Descartes' 'esprits reflechis'.'* 
Across the centuries the changing nuances of word 
meanings make it impossible to catch the exact conno- 
tation intended by an author, but Descartes' interest 
in the reflection of light rays suggests that this may 
have been the analogy he had in mind. 

A mechanism for the mediation of involuntary 
movements was not the only one for which physiolo- 
gists were .searching. The early workers were much 
exercised by what they termed 'the sympathy of parts' 
for they recognized an integration of body mecha- 
nisms that eluded nervous influence flowing only from 
the brain. Some suggested an interaction taking place 
peripherally in a plexus, an anastomosis of the 
sensory and motor nerve endings. Winslow (178), 

176. Ibid., chapt. 6. 

177. B.'\GLivi, Giorgio (1668-1707). De fibra moirue. 1700, 
book I, chapt. 5. 

178. Winslow, James Benignus (1669-1760). Exposition 
anatomique de la structure du corps humain. Paris: Duprez 
and Desessartz, 1732, pt. VI (illustrated by plates from 
Bartolomeo Eustachius (i 520-1 574). Tabulae anatomicae. 
Rome: Gonzaga, 171 4); English translation by G. Doug- 
las. Edinburgh: Donaldson & Elliot, 1772. 2 vol. 



" In discussing the sensation of outward movement of an 
eyeball the external rectus of which is paralyzed, he says, "We 
feel, then what impulse of the will, and how strong a one, we 
apply to turn the eye to a given position." von Helmholtz, 
H. Handbuch der physiologischen Optik. Leipzig: Voss, 1867, 
parts translated into English by William James in his Principles 
of Psychology. 

" Descartes used this term only once, in Passions de I'Ame. 



32 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



working in Paris and later in Copenhagen, thought 
he had found the clue in the ganglia of the sympa- 
thetic chain. These he envisaged as small brains in 
wliich intercommunication between nerves could take 
place, efTecting sympathy between various visceral 
organs. "These ganglions . . . may be looked upon," 
he said, "as so many origins or gcrmina dispersed 
through this great pair of nerves, and consequently as 
so many little brains." This ingenious but erroneous 
theory has left its name on the structures, the sympa- 
thetic ganglia. Winslow illustrated his te.xt with the 
fine plates of Eustachius that had lain for so long un- 
noticed in the \'atican Library. These plates do not 
however show the 'small brains.' 

In following the early ideas about 'sympathy be- 
tween the parts' it must be remembered that, although 
so much emphasis was laid on the humors by early 
physiologists, endocrines were unknown and conse- 
quently their influence could not be in\oked. There 
were, however, all down the centuries, some who held 
that the blood was the great integrator. In the 
eighteenth century, for example, John Hunter (179) 
was teaching that the blood was the agent of sym- 
pathy.'^ He was drawn to this view from his work on 
inflammation and fevers arising from gunshot wounds 
in the soldiers he cared for as an army surgeon in the 
Seven Years' War with France. 

Only slowly did the concept of reflex activity gain 
ground. Hunter's contemporary and fellow Scot, 
Robert VVhytt, was accumulating observations and 
making experiments that are fundamental to modern 
physiology, although his descriptions of them are also 
often cloaked by his terminology. In the first place 
(180), he recognized the in\oluntary nature of pupil- 
lary contraction and dilation and demonstrated the 
dependence of this action on the integrity of the 
corpora quadrigemina, thus anticipating the work of 
Herbert Mayo (181) in the next century. He went on 

179. Hunter, John (1728-1793). Trealise on the Blood, Inflam- 
mation and Gunshot Wounds. London : Nicol, 1 794. 

180. Whytt, Robert (17 14- 1766). An essay on the vital and 
other involuntary motions of the animal. Edinburgh: Hamil- 
ton, Balfour and Neill, 1751. 

181. Mavo, Herbert (1796- 1852). Anatomical and Phynological 
Commentaries. London; Underwood, vol. I, 1822; vol. II, 
■ 823. 

"Samuel Taylor Coleridge's comment on some of John 
Hunter's writings is perhaps a little harsh: "The light which 
occasionally flashes upon us seems at other times, to struggle 
through an unfriendly medium, and even sometimes to suffer 
a temporary occultation." Coleridge, S. T. Hints towards the 
Formation oj a more Comprehensive Theory of Life. Philadelphia: 
Lea & Blanchard, 1848. 



to the study of in\oluntarv movements of voluntary 
muscle systems in decapitated animals. The move- 
ments of animals after their heads had been severed 
was common knowledge to every housewife who had 
ever killed a chicken and had attracted the attention 
of scientists since Leonardo's day. Even in the seven- 
teenth century Boyle C'82) had recognized the impli- 
cations of these phenomena, realizing that "these may 
be of great concernment in reference to the common 
doctrine of the necessity of unceasing influence from 
the brain, being so requisite to sense and motion." 
Boyle's curiosity about the i^rain and its workings was 
interwoven with his great interest in theology, al- 
though his views on the latter did not please the 
theologians. Dean Swift was even moved to parody 
them in a satire called A Pious Meditation upon a Broom- 
stick in the Slv/e of t/ie Honourable Mr. Boyle. 

Glis.son (62) had also distinguished between 'willed' 
mo\ements and those of decapitated animals. He 
thought the latter analogous to a class of movements 
depending on a lower form of perception not reaching 
the mind. One might become aware of them (^perceptio 
sensitiva) but the\' were not ruled by the mind as were 
\'oluntarv mo\'ements Qierceplio perceptioms^. 

Whytt's experiments (183) carried the argument 
farther for he showed that this type of in\oluntary 
motion could not be explained as due to the innate 
irritability (jf muscle tissue (Haller's vis insita~), for 
preservation of the spinal marrow was essential for it. 
He was, however, not the first to discover that the 
spinal cord was essential for this type of movement. 
He had been anticipated by the Reverend Stephen 
Hales, whose many and brilliant physiological experi- 
ments make one wonder how^ much time he gave to 
his parishioners in Teddington. Whytt gives full credit 
to Hales, for he says, "The late reverend and learned 
Dr. Hales informed me that having many years since 
tied a ligature about the neck of a frog to prevent any 
effusion of blood, he cut ofT its head ... the frog also 
at this time moved its body when stimulated, but that 
on thrusting a needle down the spinal marrow, the 
animal was strongly convulsed and immediately after 
became motionless." Alexander Stuart (184) repeated 

i8a. Boyle, Robert (1627-1691). Considerations touching on the 
Usefulness of Experimental Natural Philosophy. London, 
,663. 

183. VVh^tt, R. Observations on the .Xalure, Causes and Cure of 
those Disorders which are commonly called .\ervous. Hypo- 
chondriac, or Hysteric, to which are prefixed some remarks on the 
sympathy of the nerves. Edinburgh : Balfour, 1 765. 

184. Stuart, A. Three lectures on muscular motion, read before 
the Royal Society in the year MDCCXXXVHI. London: 
Woodward, 1739. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 33 



^r<u>.3 ■ 




FIG. ii. Lejt: Alexander Stuart's experiment contirming the observations of Stephen Hales that a 
decapitated frog convulses on being pithed and then becomes immobile. (From Stuart, A. Crooman 
Lectures 1738. London: Woodward, 1739.) Ri^ht: Robert Whytt whose experiments demonstrated 
reflex action in decapitated animals and the eflTects of spinal shock. (From the portrait in the Royal 
College of Physicians, Edinburgh, by courtesy of Mr. G. R. Pendrill.) 



and confirmed this experiment and described it in a 
lecture to the Royal Society in 1738. 

Whytt in his experiments on the frog came very 
close to defining the segmental reflex. He also noted 
spinal shock, for he remarked that a decapitated frog 
could not be made to move immediately after transec- 
tion although if one waited about 15 min. it would 
react to stimuli. But perhaps the most striking of his 
observations is the one in which he anticipated 
Sherrington in regard to the stretch reflex. "Whatever 
stretches the fibres of any muscle so far as to extend 
them beyond their u.sual length, excites them into 
contraction about in the same manner as if they had 
been irritated by any sharp instrument, or acrid 
liquor" (183, p. 9). 

With the publication of Whytt's work physiologists 
were divided between regarding the movements of 
spinal animals as a lingering in the cord of powers 
originally derived from the brain, and the view that 
the spinal marrow itself was capable of sensation and 
movement. Whytt inclined to the latter view in his 
explanation of the writhings of decapitated and 
eviscerated snakes. "We are naturally led to con- 
clude," he said, "that they are still in some sense alive, 
and endued with feeling, i.e. animated by a sentient 
principle." 

Before the end of the century, Whytt's publications 
had been followed by thase of Unzer (185), of Halle 

185. Unzer, Johann August (i 727-1809). Ersle Griinde einer 
Physiologie der eigenltchten thierischen Nairn thicrischer hovper 



and of his pupil Proehaska (186) who was a practising 
ophthalmologist in Prague. Both these men contrib- 
uted more in systematization and formulation at the 
conceptual level than in the addition of new experi- 
mental facts. In England, the Sydenham Society gave 
Ijoth their books to the same translator, Thomas 
Laycock (the teacher of Hughlings Jackson), and 
through him the word reflexion became the accepted 
term. Unzer postulated several sites where reflexion 
of impressions might take place — in the brain, in the 
ganglia, in bifurcations of nerves and in plexuses. Only 
if they reached the brain would these impressions be 
consciously perceived. Unzer in discussing automatic 
movements protected himself against the attacks en- 
countered by soine of his predecessors by saying that 
"the animal machines are mysteriously and inscru- 
tably endowed by the Creator." 

Proehaska, with one foot in the past, believed in a 
sensorium commune where automatic reflexion took 
place and thought this might be in the medulla or the 
cord but did not agree with Unzer that reflexion 
might be at the level of the ganglia. He reverted to 



Leipzig: Wiedmanns, 1771; English translation by T. 
Laycock. Principles of a Physiology of the Mature of Animal 
Organisms. London: Sydenham Society, 1851. 
186. Prochaska, JiRi (1749-1820). Part III: De functionibus 
systematis nervosi, et observationes anatomico-pathologi- 
cae. In: Adnotationum Academicarum . Prague: Gerle, 1784; 
English translation by T. Laycock. Dissertation on the 
Functions of the Nervous System. London: Sydenham Society, 
1851. 



34 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



Afinlifsts 

of 

Tlir Iliustiillir iVu.,u.s Sj/slem. 




FIG. 22. Lffl. Marshall Hall. Right: one of his experiments 
to demonstrate the three parts of the reflex arc. The arc was 
broken by any of the following procedures: a) skinning the 
extremity (at 5) (the 'esodic' nerves); A) sectioning of the 
"brachial or the lumbar or femoral nerve leading to the point 
irritated" (i.e. the exodic nerve' at 2); or c') removing the spinal 
mcirrow (the spinal centre')- (From Hall, M. Synopsis of the 
Diastalttc J^'ervous System: being outlines of the Croonian Lec- 
tures delivered at the Royal College of Physicians in April 
1850.) 



the idea of an inherent vis nervosa in the nerves that 
enabled them to function in isolation from the brain 
and he supported this argument by citintr the move- 
ments of anencephalic monsters. In his view the 
■purpose' of reflex activity was preservation of the 
individual. 

Here the history of reflex activity rested for nearly 
30 years and the next advance was a technical rather 
than a conceptual one. This was the perfection by 
Legallois (187) of a method for the artificial respira- 
tion of mainmals and from then on, in many labora- 
tories, heads began to fall. Legallois, by sectioning the 
neuraxis serially from above and from below, nar- 
rowed the center of activity drastically and was so 
impressed by the amount of sensorimotor function 
left in a segment that he rather sweepingly concluded 
that the spinal cord was the principal seat of sensation 
and the source of voluntary motion. Although this 
extreme view did not gather many adherents, it was 
clear that the spinal cord could no longer be thought 
of as a mere prolongation and bundling together of 
peripheral nerves. On the contrary, the tendency now 

187. Legallois, Julien Jean Cesar (1770-1814). Experiences 
sur la principe de la vie, notamment stir celui des mouvemenls 
du coeur, el sur le siege de ce principe. Paris: D'Hautel, 1812. 



was to regard it as a caudal extension of the brain. 
Legallois should be remembered for being the first to 
recognize clearly that the respiratory center lay in the 
medulla oljlongata. 

This was the setting of the stage for the man who 
lifted the whole subject of reflex activity into the 
framework of modern neurophysiology and into 
clinical science. Marshall Hall, an Englishmen edu- 
cated in the great school at Edinburgh where he was 
a pupil of the third Monro, was a successful practising 
physician who set up a laboratory in his own house 
(in Malet Street where the present buildings of 
London L'niversity stand). Here he worked on his 
animals, mostly frogs and reptiles, collating his obser- 
vations (188) with those he made on patients (189). 
His acumen enabled him to perceive several details 
that had escaped his predecessors. For example, the 
writhings of the decapitated snake that had led Whytt 
to a postulate of lingering 'life' within the cord were 
recognized by Hall as motor responses to the renewed 
sensory stimuli set up by each movement 

Like Unzer, Hall in his work on the machine-like 
movements of decapitated animals protected himself 
from onslaught by stating them to be "all beautiful 
and demonstrative of the wisdom of Him who fashion- 
eth all things after his own Will." Hall, again like 
LTnzer, realized that the sensory impression that set 
ofl' a reflex need not be consciously perceived, al- 
though he was consistently remiss in acknowledging 
the contributions of his predecessors. He also ignored 
the work of his contemporaries, for nowhere does he 
refer to the great blossoming of knowledge of nerve 
physiology that was taking place at this time and 
which has been reviewed in an earlier section of this 
essay. He seems also to have ijeen unaware of the con- 
tractility of involuntary muscle although Baglivi 
(190) over a hundred years before he had made the 
distinction between smooth and striated muscle. Hall 
had many detractors who vigorously accused him of 
plagiarism, both from Miiller and from Prochaska. 
The first challenge was easier to meet than the second, 
for Hall's earliest communication (191) antedated 
Miiller's publication (94) on decapitated animals by 
one year. In the published report of this first paper, 

188. Hall, Marshall (1790-1857). .\'ew Memoir on the Nerv- 
ous System. London, 1843. 

189. Hall, M. Diseases and Derangements of the Nervous System. 
London: Bailliere, 1 841. 

190. Baglivi, Giorgio (1668-1707). Opera omnia medico- 
praclica et anatomica. Leyden: Anisson & Posuel, 1704. 

191. Hall, M. On a particular function of the nervous system. 
Proc. Z^ol- •^O'^- part 2, p. 189, Nov. 27, 1832. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



35 



which Hall gave to the Zoological Society of London 
in November 1832, there is, however, no full descrip- 
tion of the reflex arc nor does he use these terms. The 
emphasis is on "a function of the nervous system . . . 
distinct from sensation and \oIuntary or instinctive 
motion," being a "property which attaches itself to 
any part of an animal, the corresponding portion of 
the brain and spinal marrow of which is entire." 

The attack was pursued by others-" with great 
bitterness and its leaders engaged in such unworthy 
acts as checking on library slips to prove that Hall had 
borrowed Prochaska's book. (The slips however post- 
dated Hall's original publications.) To the modern 
worker the battle seems puerile and undignified and 
one regrets that its protagonists did not spend the 
time on experiment instead of polemics.-' Of the men 
for whom priority was being claimed, Prochaska was 
dead and it is noticeable that Miiller, a truly gieat 
man, after making generous acknowledgement to 
Hall in his Handbuch stood aloof from these bicker- 
ings. 

In essence Marshall Hall's inajor contributions to 
neurophysiology were, first (192), that sensory impres- 
sions coming into the medulla spinalis had far reach- 
ing effects in the nervous system in addition to the 
segmental efTector response,'" secondly the recognition 
that although reflex activity took place at a spinal 
level it could be influenced by the wilP^ and thirdly, 
the relationship of this fact to the exaggeration of 
reflex response on removal of the brain (193). These 
are not the only areas in which he anticipated 
Sherrington. He gave a preliminary glimpse of the 
stepping reflex, "In the actions of walking in man, I 
iinagine the reflex function to play a very considerable 
part, although there are, doubtless facts which 
demonstrate that the contact of the sole with the 
ground is not unattended by a certain influence upon 
the action of certain muscles." 

Marshall Hall introduced the word 'arc' to describe 
the refle.x pathway. Many of his other terms have, 
happily, not been retained by physiologists, for he was 
a great lover of neologisms, as his definition of the 
arc shows: "the existence in Anatomy and Physiology, of 
a continuous Diastaltic Nervous Arc including an 
Esodic Nerve, the Spinal Centre and Exodic Nerve in 

192. Hall, M. On the reflex function of the medulla oblongata 
and medulla spinalis. Phil. Trans. 123: 635, 1833. 

193. Hall, M. On the true spinal marrow, and on the excito-motory 
system of the nerves. Lectures given before the Royal So- 
ciety, privately printed, 1837. 



essential relation and connection with each other — 
and of a series of such Arcs. . . ." (194). (One recog- 
nizes here that Queen Victoria had a rival among her 
subjects in the use of italics.) 

One further contribution of Hall's at the conceptual 
level should be noted. Implicit, if not explicit, in the 
theories of the earlier physiologists was the notion 
that in voluntary movement volition directed a 
nervous influence towards the individually appropri- 
ate iTiuscles. Hall pointed out that the will was inore 
teleological and less specific in its action and not 
"directed to any muscle or set of muscles, but to an 
aim, object and purpose of their contraction" (195). 
Hall's contributions were not evaluated as highly by 
his contemporaries as they have been by later physi- 
ologists, though he himself had no doubts as to how 
they should be ranked; he stated that they were the 
greatest advance in medical science since William 
Harvey. 

The iinpact of the work of the physiologists on the 
concepts of the psychologists was very great and so 
disturbing that their literature was filled with contro- 
versy for many years. Long before the concept of 
reflex acti\ity was carried into the brain by Sechenov 
to explain its higher functions, the psychologists were 
in distress over the implication for 'sensation,' for 
'consciousness' and for 'volition,' of the developing 
knowledge of spinal reflexes. The most conspicuous 
controversy was that waged between Eduard Pfliiger 
(196), von Helmholtz's successor at the Physiological 

194. H.'VLL, M. Synopsis oj the diastaltic nervous system. Crocnian 
Lectures, London, 1850. 

195. Hall, M. Memoirs on the Nervous System. London, 1837. 

196. Pfluger, Edouard (1829-1910). Die sensorischen Func- 
tionen des RUckenmarks der Wirbelthiere nebsi einer neuen 
Lehre iiber die Leitungsgesetze der Reflexionen. Berlin, 1853. 



''"Such as, for example, George, J. D. Contribution to the 
history of the nervous system. Lond. med. Gaz- 22: 40, 93, 
1837-1838. 

-' A full account of the controversy (though scarcely an 
unbiased one) can be found in Longet, F. A. Traite d'Anatomie 
de Physiologie du Systeme Nerveiix de I'Homme et des Animaux 
Vertehres. Paris, 1842. 2 vol. 

''- "But the operation of the reflex function is by no means 
confined to parts corresponding to distinct portions of the 
medulla. The irritation of a given part may, on the contrary, 
induce contraction in a part very remote." Phil. Trans. 123: 
635, 1833. 

'' "The true spinal system is susceptible of modification by 
volition. . . ." Memoirs on the .Nervous System. London, 1837, 
part 2, p. 73. (This part of the observation was anticipated 
bv Whvtt.) 



36 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



Institute at Bonn, tmd Rudolph Lotze (197), pro- 
fessor of Philosophy at Gotlingen. Back and forth the 
battle raged, swinging from physiology into meta- 
physics and back again into experiment. The argu- 
ments all centered around the problems of whether a 
spinal animal was sentient and conscious, and whether 
its movements were purposeful. Was such an animal 
intelligent? Did it have memory? Pfliiger espoused the 
idea of consciousness in the cord, Lotze denied it; 
both were dogmatic inn to neither can we look for 
advancement of knowledge of the central nervous 
system in this context. 

In the nineteenth century, while Marshall Hall was 
still alive, the nature of inhiiiition became of major 
interest to physiologists and before the end of the 
century was to have its role in reflex activity demon- 
strated by Sherrington. Although the po.ssibility of 
inhibition had been suggested by several workers, the 
actual phenomenon had first been observed (and re- 
jected as an error of experiment) by Volkmann (198) 
in 1838 in relation to the action of the vagus on the 
heart. It was again observed, and this time accepted, 
by the Weber brothers (199) in 1845. The elder 
brother, Ernst, held the joint chair of anatomy and 
physiology at Leipzig until Carl Ludwig came in 1866 
to take over the latter section and set up his famous 
institute. The technique of the classic experiment that 
established the existence of vagal inhibition was the 
stimulation by a voltaic pile of both vagi of the frog. 
Later the Webers found that unilateral stimulation 
had the same effect and they confirmed the result by 
stimulating the vagus of a cat with an induction cur- 
rent. They reported this discovery, one of the land- 
marks of nerve physiology, at the Congress of Italian 
Scientists held in Naples in 1845 (which accounts for 
their publication being in Latin rather than in 
German). This type of inhibition, like that which was 
eventually evoked to explain Bernard's (200) obser- 
vation of the influence of the chorda tympani on the 

197. Lotze, Rudolph Heinrich (1817-1881). Instinct. In: R. 
Wagner. Handwortrnbuch. pt. ■!. Brunswick: Vieweg, 184.!- 

1853- 

ig8. Volkmann, Alfred Wilhelm (1800-1871). Uber Re- 
flexbewegungen. Arch. Anat. u. Physiol. 15, 1838. 

igg. Weber, Eduard Friedrich Wilhelm (1806-1871) and 
Ernst Heinrich Weber (i 795-1878). Experimenta, 
quibus probatur nervos vagos rotations machinae gal- 
vano-magneticae irritatos, motum cordi retardare et 
adeo intercipare. Ann. Univ. Med.., Milano 20: 227, 1845. 

200. Bernard, Claude (1813-1878). Recherches anatomiques 
et physiologiques sur la corde du tympan, pour servir a 
I'histoire de I'hemiplegie faciale. Ann. med.-psychol. i : 408, 
'843- 



submaxillary blood \'essels, seemed simple to later 
physiologists faced with the complexities of inhibition 
in the central nervous system. These had to await 
exploration by Sherrington. 

An enduring interest of Sherrington and one ex- 
haustively explored by him in the laboratory was re- 
ciprocal inner\ation of antagonist muscles, and many 
of his publications were on this subject. The attempt 
of Descartes (25) in the seventeenth century to reach 
an explanation based on channeling of vital spirits 
had no immediate successor. In the early part of the 
nineteenth century Charles Bell (201) had postulated 
the existence of peripheral inhibition by insisting on 
the need for nerves which had the opposite of an 
excitatory effect on muscle. "The nerves," he said, 
"have been considered so generally as in.struments for 
stimulating the muscles, without thought of their act- 
ing in the opposite capacity, that some additional 
illustration may be necessary." He went on to describe 
an experiment in which contraction of a flexor muscle 
coincided with imposed relaxation of its opponent 
extensor. 

The possibility of a peripherally exerted inhiljition 
of muscle contractility attracted many people at 
about this time. One of the earliest was a Dr. West 
(202) of Alford in Lincolnshire (who had heard Bell's 
lectures at the Royal College of Surgeons). \Vest's 
suggestion was that contraction was an inherent prop- 
erty of muscle and that the action of the nerve supply- 
ing it was not to evoke, but to 'restrain' or 'rein' this 
innate tendency to contract. He explained a volun- 
tary contraction as a withdrawal of this nervous re- 
straint "so as to allow the peculiar property of muscu- 
lar fibre to shew itself." The publication of West's 
hypothesis provoked some expostulation, one anony- 
mous correspondent saying this was "certainly one of 
the clumsiest contrivances that nature was ever 
accused of" The mechanism of rigor mortis was not 
understood at this time and West felt that his theory 
off"ered a possible explanation. The idea was also 
present in the arguments of many others, for example 
those of Engel (203), of Stannius (204) and of Duges 

201. Bell, C. On tiio ner\es of the orbit. Phil. Trans. 113: 289, 
1823. 

202. West, R. Uvedale. On tlic inHucnce of the nerves over 
muscular contractility. Ryan's Med. Surg. J. 1 : 24, 245, 
445, 1832. 

203. Engel, Joseph. Uber Muskelreizbarkeit. ^Ischr. Gesellsch. 
Arize, yVien i : 205, 252, 1849. 

204. Stannius, Hermann (1808-1883). Untcrsuchungen iiber 
die Leistungsfahigheit der Muskeln u. Todtenstarre. 
Vierordt's Arch, physwl. heilk. i, 1852. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



37 




f). OO If! 



Tiglira Murculi [eamdum auto- "''■'"'■ ^-^ 





FIG. 23. Lc//.- Descartes' sketch of reciprocal muscles of the eye (/Jf Humine, the Latin translation by 
Schuyl). Center: a redrawing showing closure of valves on relaxation, opening on contraction to allow 
animal spirits to How in and swell the muscle QL' Homme, the French edition of 1677). Right: Sherring- 
ton's diagram of the connections and actions of two cells of a dorsal root ganglion. The plus sign 
indicates that at the central synapses the afferent impulses excite the ipsilateral flexor muscle and 
the contralateral extensor, while inhibiting the ipsilateral extensor and the contralateral flexor 
muscle. (From Sherrington, C. S. The Integratwe Ac/inn nf Ike Nervous Svslem, 2nd ed. Cambridge: 
Cambridge, 1947.) 



(205) in Montpellier. The latter favored a peripherally 
exerted nervous influence acting against an inherent 
elasticity of muscle. 

In 1868 Hering (206) and Breuer(207) found in the 
respiratory system a parallel to Bell's experiment 
whereby distention of the lung acting through the 
pulmonary branch of the vagus inhibited inspiration 
while exciting expiration, the well-known Hering- 
Breuer reflex. And in 1883 Kronecker (208) working 
on the swallowing reflex in Ludwig's laboratory with 
his American pupil, Meltzer, demonstrated the in- 
hibitory action of the superior laryngeal nerve on in- 
spiratory muscles during contraction of expiratory 
ones. The reflex nature of .swallowing had been recog- 

205. Duces, Antoine. Traile de Physiologie Comparee de I'homme 
et des Animaux. Montpellier & Paris, 1838; Compt. rend. Sac. 
de biol. March 17, 1847. 

206. Hering, Karl Ewald Konstantin (i 834-1918). Die 
Selbststeuerung der Athmung durch den Nervus Vagus. 
Silzber. Akad. Wiss. Wien 57; 672, 1868. 

207. Breuer, Joseph (1842-1925). Die Selbstseuerung der 
Athmung durch den Nervus Vagus. Sitzber. Akad. Wiss, 
Wien 58: 909, 1868. 

208. Kronecker, Karl Hugo (1839-1914) and Samuel 
James Meltzer (1851-1920). Der Schluckmechanismus, 
seine Erregung und seine Hemmung. Arch. Anat. Physiol. 
Suppl. : 328, 1883. 



nized by Marshall Hall (195) in 1823 and the direct 
afferent nerve for it had been identified by Magcndie 
(209) to be the glossopharyngeal, but the reciprocal 
effect had not been noted by them. 

It is the fact that there are no inhibitory nerves to 
vertebrate skeletal muscle that drew the whole subject 
of reflex inhibition into the central nervous system. 
With the realization that reflex inhibition had its site 
in the central nervous system, attention was turned to 
the connection between the incoming sensory element 
of the arc and the motor component, to the junction 
between them, in other words, to the synapse (Sher- 
rington's word). That there might be an interaction of 
a synaptic kind between neurons in the periphery had 
occurred to several workers, one among whom was 
Sigmund Freud (210). His work on fresh-water crabs 
and his illustrative sketches of how he conceived of 
intercommunication between the axons of their 
ganglia came close to what is now termed an ephapse, 
although he pictured transverse crossings that sug- 
gest a uniting of fibers rather than a contiguity. 

209. Magendie, F. Lei^ons sitr les fonctions du systhne nerveux. 
Paris, 1839. 

210. Freud, Sigmund (1856- 1939). Uber den Bau der Nerven- 
fasern und Nervenzellen beim Flusskrebs. Sitzber. Akad. 
Wiss. Wien 85: 9, 1882. 



38 



HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 




.<V. DIAGRAMS ILLCSTHATrNG THE ELEMENTARY 
COMBINATIONS OF THE NERVOUS SYSTEM. 



r A 



FIG. 24. Above: Schema of the connections between the posterior and an- 
terior roots of the spinal cord as taught to students in the days before the 
neuron doctrine and the theory of the synapse. [From Bernard, C. Lemons 
sur la Physiologie el la Palhologie du Sysleme .Nerveux. Paris: Balliere, 1858.) 
Right: Connections in the nervous system as taught to students in 1885. 
(From Pye-Smith, P. H. Syllabus of a course of lectures on Physiology delivered at 
Guy's Hospital. London: Churchill, 1885.) 




Recognition of the synapse could come only after 
the neuron theory had replaced the reticular theory. 
According to the latter, strongly championed by 
von Gerlach (211), nerve cells were connected with 
each other by a diffuse fibrillary network forming an 
anastomosis. This hypothesis received support from 
Golgi (212), although it was his silver staining tech- 
nique in the hands of Ramon y Cajal (213) that 
finally disproved it, for Ramon y Cajal established 
that both axons and dendrites had free endings. To- 
gether they shared the Nobel prize in igo6, Golgi 
devoting his address to an attack on the neuron theory 
that his fellow prize winner had done so much to up- 
hold. In modern times, the synapse (an abstraction) 
is having to be remodelled in the light of what the 
electron microscope is revealing. 

The nature of central inhibition, a still incompletely 

211. VON Gerlach, Joseph (1820- 1896). The spinal cord. In: 
S. Strieker, A Aianual of Histology (English translation). 
London: New Sydenham Society, 1872. 

212. Golgi, Camillo (1844-1926). Atti Soc. ital. progr. sc. 
3rd reunion. 1910. 

213. Ramon y Caj.\l, Santiago (1852- 1934). Neuron theory 
or reticular theory. Arch. Jisiol. 5, igo8; translation by 
Purkiss and Fox. Madrid, 1954. 



resolved issue, has e\oked many hypotheses. Among 
them, those depending on mutual interference of 
impulses at the effector component of the reflex arc 
form one class. An example is the schema suggested 
by Rosenthal (214) in 1862 to explain the effect of 
efferent vagus fibers on the respiratory center. He 
proposed that an effector system excited into action 
by one nerve could have the pulsating rhythm of its 
nervous supply disturbed by inflow from another 
nerve, the result being a redistribution of previously 
grouped impulses into more frequent but less powerful 
(and hence inadequate) discharges. Lack of evidence 
for a pulse-like time-rhythm in nerve trunks led to the 
rejection of this hypothesis by W'undt, Sherrington 
and others. 

In the 1870's and 1880's attempts to explain inhi- 
bition on metabolic effects depending directly on the 
cell's response to stimulation being an assimilation of 
chemical nutrients were espoused by Gaskell (215) 

214. Rosenthal, Joseph. Die Atembeweg und ihre Bezichung zum 
nervus Vagus. Berlin, 1862. 

215. Gaskell, Walter Holbrook (1847-1914). On the 
rhythm of the heart of the frog and of the nature of the 
action of the vagus nerve Phil. Trans. 173; 993, 1882. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



39 



(for the vagus) and Hering (216) (for black-white 
sensations of the visual sense), by Vervvorn (2 1 7) (in 
his Biogenhvpothese). The hypothesis did not survive 
for long. As Forbes (218) said in his critique, "To 
assume that increase of anabolism necessarily implies 
decrease of catabolism, is to suppose that increasing a 
man's salary ensures decrease of his expenditure." A 
theory of immobilization of ion transfer during inhi- 
l)ition was propo.sed by Macdonald (219) in 1905, at 
a time when the release of potassium from injured 
nerves was receiving considerable attention. 

With the discovery of the refractory period in nerve 
[by Gotch and Burch (220) in 1889] there was some 
tendency to regard block of conduction due to excita- 
tory impulses arriving during refractoriness caused by 
preceding excitation to be the mechanism of inhibi- 
tion. This is now recognized as a misuse of the term, 
and in fact Sherrington's demonstration that after 
discharge persisting after cessation of excitation could 
be cut short by inhibitory nerve action was an early 
salutory corrective. 

In the course of researches on the inexhaustibility 
of nerve, a subject which engrossed the early electro- 
physiologists, Wedensky (221) found that a rapid 
series of strong stimuli would fail to produce more 
than a single twitch if the transmission from nerve to 
muscle were blocked either by fatigue at the end plate 
or by artificially impairing a section of the nerve by 
narcosis. If however the frequency or the strength of 
the tetanus were then reduced, the muscle went im- 
mediately into tetanic contraction. Wedensky con- 
cluded that the nerve was inexhaustible and that the 
phenomenon was one of inhibition. This may, how- 
ever, be regarded as a special usage of the term since 
the effect he observed was merely a characteristic of 
the relative refractory period of nerve and its time 
course as related to strength of stimulus (222). 

It was Sherrington's insistence on a central site for 

216. Hering, Heinrich Ewald (1866-1948). Zur Thcorio dc 
Vorgange in der lebendigen Substanz. Lotos g: 35, 1889; 
translated in Brain 20: 232, 1897. 

217. Verworn, Max. Die Biogmhypothcse. Jena: Fischer, 1903. 

218. Forbes, Alexander. Reflex inhibition of skeletal muscle. 
Quart. J. Exper. Physiol. 5: 149, 1912. 

219. Macdonald, J. S. The structure and function of nerve 
fibres. Proc. Roy. Soc, London, ser. B 76: 322, 1905. 

220. Gotch, F. and G. J. Burch. The electrical response of 
nerve to two stimuli. J. PhysioL 24: 410, 1899. 

221. Wedensky, Nicholai Yevgenevich (1852- 1922). Die 
Erregung, Hemmung und Narkose. .Arch. s^es. Physiol. 
100: I, 1903. 

222. Adrian, E. D. Wedensky inhibition in relation to the 
"all-or-none" principle in nerve. J. PhysioL 46: 384, 1913. 



the inhibitory mechanisms of skeletal muscle that 
emphasized the reflex nature of inhibition. The con- 
tributions of Sherrington and his school are the basis 
of modern ideas of the reflex at the spinal level. A 
great number of findings (223-227) made by Sherring- 
ton and brought together into a unifying explanatory 
scheme included the following major observations: that 
postural tonus of a muscle is dependent not only on 
efferent nerves but on afferent nerves from that muscle 
itself, the stimulus to the latter being from stretch re- 
ceptors [the myotatic reflex (223)]; that decerebrate 
rigidity (224) is an e.xaggerated muscle tonus in the 
antigravity muscles — a reflex standing ["an harmo- 
nious congerie of stretch-reflexes" (225)]; that the 
afferent nerve from a given muscle can elicit a con- 
traction in that muscle itself (228), without involve- 
ment of the opposing muscles of the joint;-'' that the 
main stimulus for the stepping reflex (229) does not 
come from contact of the foot with ground, as might 
be expected;-" that stimulation causing fle.xion in one 

223. LiDDELL, E. G. T. AND C. S. SHERRINGTON. ReflcXCS in 

response to stretch (myotatic reflexes). Proc. Roy. Soc, 
London, ser. fi 96: 212, 1924. 

224. Sherrington, C. S. Cateleptoid reflexes in the monkey. 
Proc. Roy. Soc, London, ser. B 60: 41 I, 1897. 

225. Sherrington, C. S. Problems of muscular receptivity. 
Linacre Lecture. Mature, London 113: 732, 892, 929, 1924. 

226. Sherrington, C. S. Selected Writings of C. S. Sherrington, 
edited by D. Denny-Brown. London: Hamish Hamilton, 
1940. 

227. Sherrington, C. S. Note on the knee-jerk and the corre- 
lation of action of antagonistic muscles. Proc. Roy. Soc, 
London, ser. B 52: 556, 1892-3. 

228. Sherrington, C. S. On reciprocal innervation of an- 
tagonistic muscle (eighth note). Proc Roy. Soc, London, ser. 
B 76: 269, 1905. 

229. Sherrington, C. S. Flexion-reflex of the limb, crossed 
extension-reflex, and reflex stepping and standing. J. 
Physiol. 40: 28, 1 910. 

''* From a series of 1 4 articles by Sherrington on reciprocal 
innervation stretching over the years from 1893 to 1909 (and 
developed in many other of his writings), the following excerpt 
may be quoted as one of his crucial experiments: "All the 
nerves of the limb being severed, except those of the vasti and 
crureus, the animal is inverted and the knee then gently but 
fully extended by raising the foot, the thigh being held vertical. 
The foot is then released, the anticrus falls, and in doing so is 
seen to be suddenly checked by exciting a contraction of the 
extensor of the knee. This contraction is different from a knee- 
jerk, for it only slowly passes off." Sherrington, C. S. Proc. 
Roy. Soc, London, ser. B 76: 283, 1905. 

'' ". . .in the intact animal (cat, dog), severance of all the 
nerve trunks directly distributed to all four of the feet up to and 
above the wrists and ankles impairs walking so little £is to make 
it highly unlikely that the loss of receptivity of the feet destroys 
any large factor in the reflex basis of these acts ' (235). 



40 



HANDBOOK OF Pin'SrOLOGY 



NEUROPHYSIOLOGY I 



In 




K^\ )> 



"/ 




' 'I '- y 

Fio. 25. Charles Scott Sherrington, from the drawing by Reginald Eves (reproduced by permis- 
sion from Selected Writings of Sir Charin Sherrington, edited by D. Denny Brown. New Vorii: Hoeber, 
1940). Right: Sherrington's classic picture of the areas for the scratch reflex in the dog. (From 
Sherrington, C. S. The Integrative Action of the Nervous System. Cambridge: Cambridge, 1947.) 




limb frequently evokes an exten.sor movement in the 
contralateral homologous limb [the crossed-extensor 
reflex (229)]; that this reflex can also be centrally 
inhiljited; and that after prolonged inhibitory stimu- 
lation there is, on withdrawal of the stimulus, an in- 
crease of contraction ['reflex reljound' (230)]. These 
are only a few of the reflex phenomena that received 
elucidation through .Sherrington's work. 

Out of a vast numljcr of laboratory experiments 
grew his unifying hypothesis of reflex excitation and 
reflex inhibition, and hence of an interdependence of 
reflex arcs resulting in an integrative action of the 
nervous system. .Sherrington's clas.sic book bearing 
this title was published (231) when he was Professor 
of Physiology at Liverpool University and was based 
on lectures he gave at Yale University. The concepts 
of 'the final common path,' of 'synaptic connections,' 
of 'central inhilaition,' of 'central excitation" and of 
'reciprocal innervation' are incorporated in modern 
ph\siology which recognizes its deln to Sherrington. 
The nineteenth century which had opened with only 
one method for tracing fiber tracts — that of dissecting 
them out as Bichat had done — gave to physiologists 
two great new tools, the histological method of 

2'50. Sherrington, C. S. Strychnine and reflex inhibition of 
skeletal muscle. J. Physiol. 36: 185, 1907. 

231. Sherrington, C. S. The Integrative Action of the .\ervous 
System. New York; Scribners, 1906; new edition. Cam- 
bridge: Cambridge, 1947. 



Wallerian degeneration and the technique of electrical 
recordings. In the hands of \'ictor Horsley and his 
associates, Gotch, Beever, Schafer and others, electro- 
physiology of spinal-cord systems made great advances 
which can be followed in the series of papers pub- 
lished in the Philiisoji/itcal Transactions between 
1886 and 1 89 1. An overall view of what could be 
achieved by this new method is given in the Croonian 
Lecture of Gotch and Horsley in 1891 (232). 

Towards the end of the century these techniques 
were being applied, not only by Horsley, but by many 
of his contemporaries to the study of the physiology 
of the brain. 



PHYSIOLOG^■ OF THE BR.MN : DEVELOPMENT OF 
IDE.'SiS AND GROWTH OF EXPERIMENT 

At the mid-eighteenth century, scientists seeking 
knowledge of the brain could look back on a history 
of their field that revealed a gradual evolution of 
anatomical knowledge about its structure but only 
conjecture about its physiology. 

Among the early Greeks the teachings of Plato had 
placed man's rational faculties where we would put 

232. Gotch, F. .\nd Victor Horsley. On the mam- 
malian nervous system, its functions and their localization 
determined by an electrical method. Phil. Trims. B 182 
267, 1 89 1. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



4' 





*j.'~ 







J..'^*— >^ ..«»|o1.lCr'Jv.-'l>*-'"'' 



FIG. j6. /,('//• th<" tliree ventricles of the brain as ens isaged by Albertus Magnus. Right: Leonardo 
da Vinci's wax cast, the first CNperimental determination of their shape. 



them now, in the head; the passions he put in the 
spinal marrow relating them to the heart, and the 
lower appetites were given a place in the cord below 
the diaphragm where they could play upon the liver. 
For Plato these were the divisions of man's tripartite 
soul. 

Under the influence of Galen the spinal nervous 
system lost this position of importance, for according 
to his doctrine other organs of the bod\', the liver and 
the heart, were the primary sites for manufacture and 
transmutation of the spirits. From the Islamic physi- 
cians came the emphasis on three ventricles with 
different functions, an anterior ventricle being the 
receiver of all incoming spirits, a 'sensus communis,' 
whereas a posterior ventricle formed the reservoir for 
the outflow of animal spirits to all muscles through 
their nerves. In a middle ventricle was to be found 
man's rea.son. Similar ideas about triple cavities in 
the brain and their allotted functions were generally 
accepted throughout the imenlightencd middle ages 
until finally an anatomist, no less a man than Leo- 
nardo da Vinci (233), mapped the true shape of the 
ventricles by pouring into them melted wax to form 
a cast. 

Throughout the sixteenth and seventeenth cen- 
turies, the structure of the brain was being unfolded 
by the anatomists but still without a parallel investi- 
gation of function. It was the cranial nerves that 

■2;j3. DA Vinci, Leonardo (1452-1519). On Ike Human Body: 
The Anatomical, Physiological, and Embryological Drawings of 
Leonardo da Vinci, with translations, emmendations and a 
biographical introduction by C. D. O'Malley and 
J. B. deC. M. Saunders. New York: Schuman, 1952. 



yielded first and Galen's seven pairs-^ (accepted on 
his authority for 1400 years) swelled to nine in the 
seventeenth century. In 1660 Schneider (234) identi- 
fied the olfactory pair and 2 years later Willis (235) 
dis.sected the accessory nerve that bears his name. 
Today's recognition of 1 2 pairs of cranial nerves dates 
from the eighteenth century and the work of von 
Soemmering (236), whose books are illustrated by 
engravings rivalled only by those of Charles Bell, 
von Soemmering wrote copiously on anatomy, illus- 
trating some of his work by his own hand and some 
by the drawings of his pupil Koeck. 

The role played by each pair of cranial nerves was 
still in soine degree obscure, for some nerves appeared 
to have more than one function, and Whytt (237) 
was one of the earlv workers to obserx'e how complex 
their action might be. He found that the optic nerve 

234. Schneider, Conrad Victor (1614-1680). Liber primus de 
catarrhis. Wittenberg: Mevius & Schumacher, 1660. 

235. Willis, Thomas (1621-1675). ^^ Anima Brutorum. In: 
Opera Omnia. Leyden : Huguetan, 1681. 

236. Soemmering, Samuel Thomas (1755-1830). /> basi en- 
cephali ei originibus nervorum cranio egredientum. Gottingen: 
Vandenhoeck, 1 778. 

237. Whytt, Robert (1714-1766). An essay on the vital and 
other involuntary motions of animals. Edinburgh : Hamilton, 
Balfour and Neill, 1751. 



^* According to Galen's numbering, the seven pairs of 
cranial nerves were: /) optic; 2) oculomotor and abducens 
taken together; 3) and 4) were both parts of what is now called 
the trigeminal, j) facial together with the auditory; 5) the 
glossopharyngeal, vagus and accessory nerves; 7) the hypo- 
glossal. 



42 



HANDBOOK OF I'H\SIOLOGY 



NEUROPHYSIOLOGY I 




FIG. 27. Thomas Willis and the illustration of the base of 
the brain taken from his book De cerebri anatome. The circle of 
Willis, named for him, had been depicted by several anato- 
mists before him. Willis was fortunate in having Christopher 
Wren as his illustrator. 

was not .solely concerned with vision but that it car- 
ried the stimulus that led to the contractile response 
of the iris to light. In the post-mortem examination 
on a child with fixed pupils he found a lesion blocking 
the inflow from the optic nerves to the thalamus and 
inferred that this impairment of sen.sory inflow was 
responsible for the motor deficit that had been the 
clinical sign. This was indeed the recognition of a 
reflex arc, and the pupillary reflex was for many years 
known by his name. 

As noted above, Willis had di.ssected the spinal 
accessory nerve to its junction with the cord but he 
believed it to convey voluntary control. Lacking a 
.scientific acumen equal to his skill as a dissector, and 
influenced by Galen, he thought this nerve anasto- 
mosed with the vagus (the "wandering' nerve). 
Schneider, on the other hand, had no doubts as to 
the action of the olfactory nerves for it was his work 
on the nasal mucosa and olfactory processes that led 
to his identification of them. Willis also was aware of 
their function for he called them the 'smelling' nerves. 
He noted that within the skull they had 'mammillary 
processes' and said, "As to the Fibres and Filaments 
or little strings stretching out from the more soft 
nerves through the holes of the Sieve-like Bone into 
the caverns of the Nose, these are found in all Crea- 
tures who have the mammillary Processes: so it is 
not to be doubted, but that these Processes, with this 
appendix and its medullary origine is the Organ of 
Smell."-' Willis called in his knowledge of compara- 

" The quotations arc from Pordagos translation (1683) of 
Willis, T. Cerebri anatome: cui acces\it nervorum descnpho el usus. 
London: Flesher, 1664. 



five anatomy and noted that "the filaments or little 
strings" of the organ of smell were "more remarkable 
in hunting Hounds than in any other Animal whatso- 
ever. 

The ner\es that had ijoth sensory and motor 
branches proved the most difficult. Magendie (238) 
at first thought the fifth nerve was sensory and nutrient 
to the face, and the seventh nerve entirely motor, 
since cutting it caused facial paralysis without reliev- 
ing neuralgia. In 1820 Charles Bell (147), dissecting 
the nerves of the face, noticed that the fibers of the 
seventh nerve went to muscle whereas those of the 
fifth entered the skin. He suspected they .served diff'er- 
ent functions, and being himself an anatomist rather 
than an experimentalist, asked his brother-in-law, 
John Shaw, to make a study of the effect of sections 
of these nerves. Using an unusual experimental 
animal, the donkey, Shaw was able to demonstrate 
paralysis in the one case, loss of reaction to touch in 
the other; neither he nor Bell whose fine drawings 
illustrate his findings recognized the mixed nature of 
these nerves. After this beginning several workers 
added their contributions to the further clarification 
of the cranial nerves, prominent among these being 
Mayo (239) (who taught the course in anatomy and 
physiology at King's College, London). 

It was only in the eighteenth century that doubt 
was first thrown on the assumption that the sympa- 
thetic trunk (or 'intercostal' nerve, as it was then 
called) was an appendage of the brain. This grew 
from the transection experiments of Pourfour du 
Petit (240) and his oijsers'ations on contraction of the 
pupil. For centuries anatomists had shown this nerve 
as stemming from the brain. V'esalius (7), in his 
drawings of the human nervous system, put it in one 
trunk with the vagus. (In the dog, though not in man, 
the two nerves lie in the same sheath in the neck 
region.) Eustachius (241) separated the two, but like 
many after him, including Willis, he depicted an 
intracranial origin. These drawings of the anatomists 
must have been designed to be consistent with Galen- 



■238. Magendie, F. J. physiol. exper. et path. 4: 176, 302, 1824. 

239. Mayo, H. Anatomical and Physiological Commentaries. 
London: Underwood, vol. I, 1822; vol. II, 1823. 

240. Pourfour du Petit, Franqois (1664-1741). Memoire 
dans lequel il est demonstre que les nerfs intercostaux 
fournissent des rameaux que portent des esprits dans les 
ncrfs. Hnt. Acad. ray. Sc. Paris i, 1727. 

241. Eustachius, Bartolommeo (1520-1574). Tabulae anato- 
micae (posthumous). Rome : Gonzaga, 1 7 1 4. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



43 



ist doctrine, rather than with observation I'rom dis- 
section, du Petit's experiments came very close to 
uncovering the action of vasomotor nerves, the suij- 
ject that was to receive so much investigation in later 
years from Claude Bernard (242, 243), from Clarl 
Ludwig (244) and from Pavlov's other teacher, Cyon 
(245). Bernard's experiments were mostly on skin 
temperature changes due to vasomotor action, al- 
though at no time would he relinquish entirely an 
explanation on a metabolic basis. Ludwig had found 
the secretory action of the lingual nerve but he did 
not separate it from the chorda tympani as Bernard 
did later. 

A marked advance in understanding the physiol- 
ogy, not only of the cranial nerves but of the brain 
itself, came when techniques were developed for 
ablating and stimulating parts of the central nervous 
system without the animal succumbing to the pro- 
cedures. The surgery in the early attempts was fre- 
quently so drastic that results were rarely specific. 
For example, the experimental results of Willis that 
confirmed his belief in the cerebelliun as a vital 
center were probably due to his animal's having suc- 
cumbed to injuries near the fourth ventricle. Other 
early experimenters such as Duverney (246) with his 
pigeons, Chirac (247) and Perrault (248) with their 
dogs had to be satisfied with very brief durations of 
survival. 

At the opening of the nineteenth century interest in 
localization of cerebral function had been widely 
stirred by the lectures of Franz Gall (249) in Vienna. 
Unfortunately Gall's reputation as a phrenologist has 

242. Bernard, Claude (18 13-1878). InHucnce du Kiand 
sympathique sur la sensibilite et sur la calorification. 
Cnrnpt. rend. Soc. de biol. 3; 163, 1851. 

243. Bernard, C. De I'influence de deux oidies dc ncifs qui 
determine les variations de couleur du sang veineux dans 
les organes glandulaircs. Compt. rend. Acad, sc, Paris 47: 

^4.5. 393. 1858. 

244. Ludwig, Carl Friedrich VVilhelm (1816-1895). Mill, 
naturjorsch. Gessellsch. ^urich 50, 1851. 

245. Cyon, Ilya (1842-1912) and C. F. \V. Ludwig. Die 
Reflexe eincs der sensiblen Nerven des Hcizcns auf die 
motorischen der Blutgefasse. Arb. Physiol. Insl., Leipzig 1 : 
128, 1867. 

246. Duverney, Joseph Guichard (1648-1730). Phil. Trans. 
Roy. Soc. 19; 226, 1697 (reported by Preston). 

247. Chirac, Pierre (1650-1732). Du niolu cordis analylica. 
Montpellier, 1698. 

248. Perrault, Claude (1613-1688). Mernoires pour servir a 
I'histoire des animaux. Paris: Acad. d. Sci., 1671-1676. 

249. Gall, Franz Joseph (1758-1828) and Johann Caspar 
Spurzheim (1776-1832). Recherches sur le systeme ner- 
veux en general, et sur celui du cerveau en particulier. 
Mem. Inst. Paris 1808. 





fc: 




fig. 28. Above: Gall and Spurzheim's map of a skull with 
certain areas marked for correspondence with different mental 
acu Ities. Below, for comparison : Gall's skull on the left, that of 
Spurzheim on the right. Although Gall's own ideas were chan- 
neled into phrenology, they were influential in directing interest 
to the study of cerebral localization. (The skull of Gall is in the 
Musee de I'Homme in Paris and is reproduced here by the 
kindness of Dr. Ardvege; that of Spurzheim is in the Warren 
Museum at the Harvard Medical School, and has been photo- 
graphed by permission of Dr. P. L Yakovlev.) 



overshadowed his more important work on the fiber 
tracts of the white matter of the brain, work which 
clarified the pre\iousl\- contradictory ideas as to the 
anatomy of the commissures and of the pvramidal 
decussation. But, while his contemporaries were con- 
cerning themsehcs with sites for sensory and motor 
functions. Gall was propo.sing localization of mental 
faculties and he may be regarded as a pioneer in 
emphasizing the importance of the grey matter for 
intellectual processes. It was when, together with his 
pupil, Spurzheim (250), he proceeded to assign 
separate 'organs' in the brain to the different mental 
faculties and to relate these to bumps on the skull 
that he isegan to be challenged. All the same, in spite 

250. Gall, F. J. and J. C. Spurzheim. Anatomic et physiologic 
du systhne nerveux en general et du cerveau en particulier, avec 
des observations intellectuelles et morales de r hotnme et des ani- 
maux, par la configuration de leur teles. Paris: Schoell 1810— 
1819 (vols. I & II by Gall & Spurzheim; vols. Ill & IV 
by Gall). 



44 



HANDBOOK OF PHYSIOLOGY '-^ NEUROPHYSIOLOGY I 




FIG. 29. Two investigators of the cerebellum, Pierre Fiourens 
(1794- 1 867) and Luigi Luciani (i 840-1 921). 



of its bizarre concepts, phrenology had a surprisingly 
wide acceptance for a considerable period even 
among the medical profession. It was to the psychol- 
ogists (although that term was not yet in use) that 
phrenology particularly appealed, for it was the first 
major consideration of mental characteristics as 
attributes of brain function. 

One of the more prominent men to attack Gall's 
doctrines was Fiourens who made a sweeping rejec- 
tion of all such ideas, denying the brain any discretely 
localized action. But Fiourens' monograph (251) 
appeared some years after the deaths of Gall and 
Spurzheim both of whom had built up comfortable 
careers out of their speciality. Fiourens recognized 
three major functional regions of the brain (the 
cerebral hemispheres, the medulla and the cere- 
bellum), but within these entities he envisaged their 
action as global and their roles as being sensory, vital 
and motor, respectively. Concerning the cerebral 
hemispheres he said that animals that survive their 
removal "lose perception, judgment, memory and 
will . . . therefore the cerebral hemispheres are the 
sole site of perception and all intellectual abilities" 
(252). He did not hesitate to infer subjective qualities 
and faculties. In one of the more renowned of his 
experiments (253) he had kept a pigeon alive after 
removal of its cerebral hemispheres. The bird was 

■251. Flourens, Pierre (1794-1867). Examai de Phremlogie. 
Paris, 1842; English translation by D. de L. Meigs. 
Phrenology Examined. Philadelphia, 1846. 

252. Flourens, P. Recherches experimentales sur les proprietes et les 
fonctions du systeme nerveux dans les animaux verlebres. Paris: 
Crevot, 1824. 

253. Flourens, P. Arcli. gen, de med. 2: 321, 1823. 



'blind' and 'deaf and appeared to be asleep although 
it stirred when poked. Flourens went so far as to say 
that the bird lost its volition and "even the faculty of 
dreaming." He noted that it retained the sense of 
equilibrium and that its pupils still reacted to light. 
Others repeating Flourens' experiments were uncon- 
vinced, for their decerebrate pigeons could be starded 
by a loud noise and could avoid obstacles. 

Since sudden death followed section of the medulla, 
Flourens concluded that here lay the essential mecha- 
nism for respiration and the maintenance of life. In 
this conclusion he had of course been anticipated by 
Legallois. Much of Flourens' fame as an experimental- 
ist derived from his observation that extirpation of the 
cerebellum (in birds and mammals) caused loss of 
coordinated movement. Flourens, who.se interest lay 
so deeply in the elucidation of the control of voluntary 
movement, was himself to suffer paralysis for a long 
period before his death. 

In the 1820's when Fluorens was pursuing these 
experiments, many workers were 'mutilating' ani- 
mals (to use Gall's phrase) (254), and some jockeying 
for priority was inevitai)le. Most of Flourens' observa- 
tions, particularly those on the cerebellum, had been 
anticipated by Rolando at Sassari, whose treatise 
(255) of 1809 (written in the Italian language and 
printed and illustrated ijy himself) was therefore re- 
published in French in an abbreviated form in 1824 
C256). 

Rolando did not succeed in keeping his animals 
alive. Even his tortoises died after removal of their 
brains, although Fontana who had been successful 
with these animals showed him his own technique. 
Many of Rolando's conclusions (257) were therefore 
incorrect since he mistook surgical shock for paralysis. 
Less ruthless extirpations, of the hemispheres only, he 
found to be compatible with life. Rolando believed 
the cerebellum to be a kind of 'voltaic pile' and the 
source of all movement. Flourens thought it merely 
the regulator. Magendie (258) disagreed, holding 
cerebellar function to be maintenance of equilibrium. 

254. Gall, F. J. Stir les fonctions du cerveau et sur eelles de chacune 
de ses parties. Paris, 1822-1825. 6 vol. 

255. Rolando, Luigi (1773-1831). Saggio sopra la vera strutlura 
del cervello delV uorno de degi animali e sopra le funzioni del 
ststerna nervoso. Sassari, 1 809. 

256. Rolando, L. Experiences sur les fonctions du systeme 
nerveu.x. J. physiol. exper. et path. 3: 95, 1823. 

257. Rolando, L. Osservazioni sul cervelletto. Mem. reale 
aecad. sc. Turin 29: 163, 1825. 

258. Magendie, F. Precis elhnenlaire de Physiologic. Paris, 1825; 
English translation by E. Mulligan. Edinburgh: C^arfrae, 
1826. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 45 





FIG. 30. Lejt: Magendie's technique for sectioning the fifth nerve in the living rabbit. The dissec- 
tion is to demonstrate the insertion of his instrument. On the rabbit's right, the probe is seen entering 
the base of the siiull and reaching the trunk of the fifth nerve at H. On the animal's left, the end 
of the instrument is seen at E and the sectioned nerve at G. (From : Bernard, C. Leqons sur la Pkysiologie 
el la Pathologif du Sysleme .^eri'eux. Paris: Bailliere, 1858. Right: pigeon deprived of its cerebral hemi- 
spheres in position described by Flourens. (From: Luciani, L. Human Physiology, English ed. Lon- 
don: Macmillan, 1915.) 



He reached this conclusion from studying the dis- 
turbance of gait in a duck-* from which he had re- 
removed the cerebellum unilaterally. He followed 
these experiments with bilateral destructions and 
noticed forced movements. The great contribution 
towards our modern knowledge of cerebellar mecha- 
nisms came from Luciani of Florence whose book // 
Cervellelto (259) is a classic, as is also his te.xtbook of 
physiology (260). 

Magendie in the obsersations he made on decere- 
brate animals (261) anticipated Sherrington by an 
accurate and detailed description of decerebrate 
rigidity in rabbits. This was in the days before the 
discovery of anesthesia and Magendie was severely 

.■59. Luciani, Luigi (1840-1921). // Cervelletto. Florence, 1891. 
260. Luciani, L. Human Physiology. English translation by F. A. 

Welby. London: Macmillan, 1915. 
■261. Magkndie, F. Sur le siege du mouvement et du sentiment 

dans la moelle epiniere. J. phvsiol. cxper. ct path. 3; 153, 

1823. 



criticized for his practice of vivisection. But extirpa- 
tion experiments on animals could give no clue to the 
cortical representation of speech. This had to come 
from clinical observation with studies at autopsy. 
Gall had placed language in the anterior lobes and the 
first clinical reports seemed to confirm this. In fact, 
the great surge of work aiming to establish localized 
centers in the human brain began with the speech 
center. In his studies of encephalitis Bouillaud (262}, 
a pupil of Magendie and later Professor of Medicine, 
had reasoned that the anterior lobes of the brain were 
necessary for speech and went on to ob.serve that 
other focal lesions of the brain caused localized im- 

262. Bouillaud, Jean Baptiste (i 796-1 881). Traite clinique et 
physiologique de V encephalitt' ou inflammation du cerveau. Paris : 
Bailliere, 1825. 



^' Sherrington in quoting this experiment mistranslated 
Magendie's word 'canard' as 'water-dog.' 



46 HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 





'i-yanuC 



Strasssui 



FIG. 31. Goltz and one of his decorticate dogs. (Studio portraits of man and dog are reproduced 
here by the tcind permission of Dr. Paul Dell.) 



pairment of mu.scular movement. The cause of cere- 
bral localization was taken up by his son-in-law, 
Auburtin (263), who predicted that a lesion would 
be found in the anterior lobes of an aphasic patient 
who was at that time in the hospital of Bicetre under 
the surgeon Pierre Broca. Autopsy confirmed Aubur- 
tin's prediction, pinpointing the lesion in the left 
anterior lobe. The next aphasic patient on Broca's 
service was found at autopsy to have an even more 
discrete lesion — in what is known to this day as 
Broca's area ('-64)- The name of Auburtin has been 
forgotten, as has Broca's term 'aphemia' for aphasia. 

Broca's speech area (the left third frontal convolu- 
tion) which he thought to be concerned with articula- 
tion was to be challenged by Pierre Marie (265) in 
the twentieth century, but the new concept of cerebral 
localization de\eloped like a wave in the later 1800's 
• — a wave that is only now beginning partially to 

263. AuBERTiN, Ernst (18-25- )■ Considerations sur les 
localisations cerebrales, et en particulier sur le siege de la 
faculte du langage articule. Ga:^. hehd. med. et chir. 10: 318, 
348, 397. 455. 1863. 

264. Broca, Pierre Paul (1824-1880). Perte dc parole, 
ramoUissement chronique et destruction du lobe anterieur 
gauche du cerveau. Bull. soc. anthropol. Paris 2: 235, 1861. 

265. Marie, Pierre (1853- 1940). Revision de la question de 
I'aphasie; la troisieme circonvolution frontale gauche ne 
joue aucun role special dans la fonction du langage. Sem. 
med. Paris 26: 241, 1906. 



recede. For the physiologists the impressive experi- 
ments were those of Goltz of Strasbourg who, after 
starting with frogs (266}, mastered the technique of 
keeping warm-blooded animals ali\e for prolonged 
periods after drastic extirpations of large portions of 
their brains (267). Three of his dogs became famous. 
The first two survived 57 and 92 days respectively, 
the third being purposely sacrificed at 18 months. 
Goltz exhibited them at international congresses, 
killed one of them before an audience and gave their 
brains to Langley in Foster's laboratory to dissect 
(268, 269). Sherrington's participation in the necropsy 
of one of these dogs was the subject of his first pub- 
lished paper (in 1884) (270). All who witnessed the 
remarkable degree of retention of sensibility and 

266. Goltz, Friedrich Leopold (1834-1902). Beilrcige z"r 
Lehre den Funktionen der Nervenz.enlren des Frosches. Berlin : 
Hirschwald, i86g. 

267. Goltz, F. L. Der Hund ohne Grosshirn. .irch. ges. Physiol. 
51: 570, 1892. 

268. Langlev, J. N. Report on the parts destroyed on the 
right side of the brain of the dog operated on by Professor 
Goltz. J. Physiol. 4: 286, 1883. 

269. L.ANGLEV, J. N. AND A. S. Grunbaum. On the degenera- 
tion resulting from removal of the cerebral cortex and 
corpora striata in the dog. J. Physiol. 1 1 : 606, 1890. 

270. Langlev, J. N. and 0. S. Sherrington. Secondary de- 
generation of nerve tracts following removal of the cortex 
of the cerebrum in the dog. J. Physiol. 5: 49, 1884. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLDGV 



47 



mobility by these animals and who later studied the 
necropsy findings from the Cambridge laboratory 
were astounded, and there can be no doubt that these 
experiments gave a great impetus to neurosurgical 
procedures in animals and in man. 

The physiology of the brain was now beginning to 
unfold and to reveal itself in dynamic terms after 
centuries of static representation in the two-dimen- 
sional pages of the anatomy books. To clinical obser- 
vation of impairment by disease states, three experi- 
mental techniques were added : regional ablation, 
stimulation (both mechanical and electrical) and 
eventualh' the recording of the brain's own electricity. 

Mechanical and chemical irritation of the cortical 
surface had suggested itself to many in\estigators down 
the years, some of the attempts reaching the extremes 
of the bizarre (see, for example, fig 32). Cabanis 
(271), the celebrated physician and ideologue, had 
provoked convulsive movements in muscle groups 
that .seemed to vary with the region irritated. Earlier, 
Haller (272), searching for irritability, had pricked 
the brain and applied irritating fluids and concluded 
that the grey matter was insensitive to stimulation 
and that the white matter was the seat of sensation 
and the source of movement. 

The Italian physiologists had been more successful. 
The Abbe Fontana (273) and Caldani (274) (Gal- 
vani's predecessor in the chair of anatomy at Bologna) 
had convulsed their frogs by electrical stimulation 
inside their brains. Rolando (255), following their 
lead, extended his experiments to pigs, goats, sheep, 
dogs and also to birds. The influential Magendie 
however had failed and had proclaimed the cortex 
electrically inexcitable; an opinion in which he was 
backed by Flourens (252). In these days before the 
neuron had been recognized as the unit of the nervous 
system, before the pyramidal fibers were known to be 
processes of cortical cells, there was no a priori reason 
to expect electrical stimulation of the cortical surface 
to have a peripheral effect, but soon an incontro- 
\ertible proof was to be given. 




FIG. 32. One of the bizarre experiments of .Mdini on two 
freshly-decapitated criminals. In the center is a voltaic pile, the 
circuit through the heads being completed by conducting 
arcs. .Mdini, Galvani's impetuous nephew, lacked the sagacity 
and scientific acumen of his famous uncle. (From Aldini, G. 
Essai Theorique el Experimental sur le Galvanisme. Paris; Fournier, 
1804. 2 vol.) 




FIG. 33. Two pioneers in attempts to stimulate the brain: 
the Abbe Fontana, physician to the Archduke of Tuscany and 
professor of physics in the University of Pisa; and Caldani, 
Galvani's predecessor in the chair of anatomy at Bologna. 
(The portrait of Fontana is reproduced by courtesy of Dr. G. 
Pupilli.) 



271. Cabanis, Pierre J.-G. Rapports du physique et du moral de 
I'homme. Paris: Bibliotheque Choisie, 1830. 

272. ZiNN, JoHANN Gottfried (i 727-1 759) and A. Haller. 
Memoir es sur les parties sensibles et trrilables du corps animal. 
Lausanne: D'Arnay, 1760. 

273. Fontana, Felice (1720-1805). Acead. Sc. 1st. Bologna, 
>757- 

274. Caldani, Leopoldo (1725-18 13). Institutiones phystologicae 
et pathologicae. Leyden: Luchtmans, 1784. 



The pioneers were Fritsch & Hitzig (275) (two 
young privatdocents in Berlin) with their now famous 
experiments in which they u.sed a galvanic current 
and from which evolved the idea of a 'motor cortex.* 

275. Fritsch, Gust.w Theodor (1838-1891) and Eduard 
Hitzig (1838-1907). Uber die elektrische Erregbarkeit 
des Grosshirns. .Irch. Anal. Physiol, miss. Med. Leipzig 37: 
300, 1870. 



48 



HANDBOOK OF Pm-SIOLOGY 



NEUROPHYSIOLOGY 1 



fXi^*^:\m^.l 




FIG. 34. Gustav Fritsch and Edouard Hitzig. (Photographs 
reproduced by kind permission of Dr. A. E. Walker, for whom 
Professor Stender of Berlin obtained the picture of Hitzig.) 



Ferrier (276-278), a few years later, in a long series 
of experiments using faradic stimulation in monkeys 
was able to bring out not merely muscle twitches of an 
indeterminate kind but also grosser movements. Of 
course, as we now know, these are imprecise and even 
athetoid in comparison with movements made by the 
animal naturally. Benefitting from the parallel devel- 
opment of electrical techniques, Victor Horsley, in a 
series of papers with Beevor (279, 280) in the next 
decade, described more closely the motor areas in the 
monkey cortex. From these experiments there emerged 
the designation of the precentral gyrus as predomi- 
nantly motor in function and the postcentral as sen- 
sory. Between the two, Beevor & Horsley (281, 

^76. Ferrier, David (1843-1928). The localization of function 
in the brain. Proc. Roy. Soc. 22: 229, 1873-4. 

277. Ferrier, D. Experiments on the brain of monkeys. P/ut. 
Trans. 165:433, 1876. 

278. Ferrier, D. The Function of Ihf Brain. London Smith 
Elder, 1876. 

279. Beevor, C. E. and V. Horsley. A minute analysis (ex- 
perimental) of the various movements produced by stim- 
ulating in the monkey different regions of the cortical 
centre for the upper limb as defined by Professor Ferrier. 
Phil. Trans. 178: 153, 1887. 

280. Beevor, C. E. and V. Horsley. A further minute analy- 
sis by electrical stimulation of the so-called motor region 
(facial area) of the cortex cerebri in the monkey. Phil. 
Trans. 185: 39, 1894. 

281. Beevor, C. E. and V. Horsley. An experimental in- 
vestigation into the arrangement of the excitable fibres of 



282) recognized an area which they called 'the zone 
of confusion.' An important point that emerged from 
their use of this technique was that in addition to areas 
of maximal representation of a given movement, the 
cortex also has marginal zones that are less specific. 
In other words, they found no sharp demarcation 
lines. 

With Schaefer (283), Horsley went on to further 
studies of both motor and sensory function, using 
ablation as well as electrical excitation. The basic 
interest was of course in the application of these find- 
ings to man, especially in the light of the observations 
of Hughlings Jackson on the march of symptoms 
during the epileptic fit (284). Species differences 
came markedly to light when Beevor & Horsley 
compared their findings on the bonnet monkey with 
those in the orangutan. The first pioneers to attempt 
electrical stimulation of the cortex in man (through 
holes in the skull) were Bartholow in America in 
1874 (285) and Sciamanna 8 years later in Italy (286). 
These were followed by Keen (287), in his youth an 
army surgeon in the American Civil War and later 
professor of surgery at Jefferson Medical College. In 
1888, in a patient whose seizures began in the hand, 
he removed the area the stimulation of which caused 
movements of the wrist. He used a 'faradic battery,' 
and with it found areas for hand, elbow, shoulder 
and face movements. When respiration and circula- 
tion became poor, he revived the patient with brandy 
injected into the forearm. In the same year several 
other workers applied a similar technique in man but 



the internal capsule of the bonnet monkey. Pliil. Trans. 
181 : 49, 1890. 

282. Beevor, C. E. and V. Horsley. A record of the results 
obtained by electrical excitation of the so-called motor 
cortex and internal capsule in the orang-utang. Phil. 
Trans. i8i : 129, i8go. 

283. Horsley, V. and Edward Albert Schaefer (1850- 
1935). A record of experiments upon the functions of the 
cerebral cortex. Phil. Trans. 179: i, 1888. 

284. Jackson, John Hughlings (1835-191 i). Unilateral 
epileptiform seizures, attended by temporary defect of 
sight. Med. Times Gaz. I : 588, 1863. 

285. Bartholow, Roberts (1831-1904). Experimental in- 
vestigations into the functions of the human brain. .Im. 
J. M.&. 67:305, 1874. 

286. Sciamanna, E. Gli avversari delle localizzazioni cercbrali. 
Arch, psichiat. Turin 3: 209, 1882. 

287. Keen, William Williams (1837-1932). Three successful 
cases of cerebral surgery including (i) The removal of a 
large intracranial fibroma; (2) Exsection of damaged 
brain tissue; and (3) Exsection of the cerebral centre for 
the left hand; with remarks on the general technique of 
such operations. Am. J. M. Sc. 96; 329, 452, 1888. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



49 



^P^^l 




P^^^ 


v.^, \_rrg<^ 


HHHHHH^^HHIi^^^Hiiii^wiKd^^-.-je^^ ^ 




FIG. 35. Victor Horsley and one of his experiments on the locaUzation of the motor cortex. (The 
latter illustration from Trans. Congr. Am. Physic. Surg, i : 340, 1888.) 



systematic exploration had to wait for Gushing, 
Foerster and Penfield in the modern age of neuro- 
surgery, and for the development of clinical neuro- 
physiological investigation. 

In the light of clinical oljser\ation and the results of 
electrical stimulation, the concept that the cortical 
grey matter acted as a whole and that motor function 
had no representation above the basal ganglia began 
to crumble. At this same period, the birth of a new 
technique brought yet another method of approach 
for the investigator. This was the recording of brain 
potentials evoked by sen.sorv stimulation and the 
discovery of the Ijrain's own electrical activity, the 
dawn of electroencephalography. 

In 1875 Richard Caton (288), at the Royal Infir- 
mary School of Medicine in Liverpool, while searching 
for the cerebral counterpart of du Bois-Reymond's 
action potential in nerve, not only found it, but 
noticed that when both of his electrodes lay on the 
cortical surface there was a continuous waxing and 
waning of potential. This oscillation of the base line 
was present in the unstimulated animal and Caton 
proved it to be unrelated to respiratory or cardiac 
rhythms. He also proved these fluctuations to be 
biological in origin ijy showing them to be vulnerable 

288. Caton, Rich.^rd (1842-1926). The electric currents of 
the brain. Brit. M. J. 2: 278, 1875. 



to anoxia and to anesthesia and to be abolished by 
death of the animal. In his first work Caton's experi- 
mental animal was the rabbit and his detecting 
instrument was a Thomson's galvanometer. This 
was in the days before photographic recording of 
laboratory observations and Caton's first publication 
of his findings took the form of a demonstration before 
the British Medical Association (289). Superimposed 
on these o.scillations Caton found potential swings 
related to sensory stimulation and realized immedi- 
ately the meaning of this for cerebral localization 
studies. Caton went on to use monkeys and gave fur- 
ther reports of his results in 1877 and in 1887 (290). 
the latter at the International Medical Congress held 
that year in Washington, D. C. 

Strangely enough, in spite of the prominent groups 
before whom Caton gas'e his demonstrations and the 
popular medical journal in which he reported them, 
his work received little attention at the time, even 
among English-speaking physiologists. Meanwhile in 
Poland, a young a.ssistant in the physiology depart- 
ment of the University of Jagiellonski in Krakow, 

289. Caton, R. Interim report on investigation of the electric 
currents of the brain. Brit. M.J. i : Suppl. 62, 1877. 

290. Caton, R. Researches on electrical phenomena of cere- 
bral grey matter. Tr. Ninth Internat. Med. Congr. 3 : 246, 
1B87. 



50 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 




FIG. 36. Richard Caton, shown in his thirties at the period 
when he was working in electrophysiology. (From a photo- 
graph in possession of the writer, being the generous gift of 
Miss Anne Caton.) 



Adolf Beck, not knowing of Caton's work 15 years 
earlier, was searching initially for the same phenom- 
enon, namely for electrical signs in the brain of im- 
pulses reaching it froin the periphery. Like Caton 
before him he succeeded, but he also found the brain 
wave. His animals were mostly dogs and he pub- 
lished the protocols of all his experiments in the 
Polish language for a doctoral thesis (291). In order 
to reach a wider audience he sent a short account to 
the most widely read journal in Germany, the 
Cenlralblatt jur Physiologie (292). A spate of claims for 
priority for finding sensorily evoked potentials fol- 
lowed the German publication of Beck's findings — 
the first coming from Fleischl von Marxow, Profes.sor 
of Physiology in Vienna (293), and the next from 

291. Beck, .Adolf ( 1863-1942). O-z'tacz^nie lokalizocyi w moz^u 
i rdzt'niu za ponwca zj^^^'i^^^ elektry czynch (Thesis). Krakow; 
Univ. Jagiellonski, 1890. 

Q92. Beck, A. Die Bestiinmung der Localisation der Gehirn 
und Riichenmarksfunktionin vermittclst der elektrischen 
Erscheinungen. Centrathl. Physiol. 4: 473, 1890. 

293. Fleischl von Marxow, Ernst. Mittheilung betrefTend 
die Physiologie der Hirnrinde (letter to the editor dated 
Vienna, Nov. 24, 1890). Centralbl. Physiol. 4: 537, 1890. 



Gotch and Horsley (294). It is noticeable that it was 
the electrical response of the brain to sensory stimula- 
tion that drew the most interest, for this was a finding 
that lay directly in the main stream of current think- 
ing about cortical localization of function. The 
completely novel idea of a continuously fluctuating 
electrical potential intrinsic to the 'resting' brain 
was, at that time, of interest only to its two independ- 
ent discoverers, Caton and Beck. 

The somewhat acrimonious wrangle over priority 
was based in Fleischl von Marxow's ca.se on work 
done in 1883. This had not been published but only 
noted down in a sealed letter which he had deposited 
with the University and which he asked to have 
opened after reading Beck's report in 1890. He was 
solely concerned with response potentials and noted 
"little or no movement of the base line." He was 
clearly unaware of Caton's reports and demonstra- 
tions. Gotch and Horsley's ignorance of their country- 
man's work is less easily understood. Caton was a 
prominent figure at Liverpool, the first holder of the 
Chair of Physiology in which Gotch was to follow him 
(and later Sherrington). 

The dispute in the columns of the Centralblalt o\er 
priority for discovery of the electrical currents of the 
brain was finally stilled by a letter from Caton (295), 
drawing the attention of the protagonists to his 
publication of 1 5 years earlier. By the turn of the 
century the electrical activity of the brain had reached 
the textijooks (296). Caton's interests had developed 
along many lines and he became prominent in 
.se\eral fields of medicine and scholarship as well as in 
public affairs, becoming in turn President of the Medi- 
cal Institution and Lord Mayor of Liverpool. Beck 
(297), who at the age of 32 became professor of 
Physiology at the University of Lvov, continued to 
work on the subject into this century, publishing with 
his old professor Cybulski, and interest was thereby 
aroused in Germany and in Russia. He met a tragic 
death during the German occupation of Poland. 

294. Gotch, F. .and V. Horslev. Uber den Gebrauch der 
Elcktricitat fiir die Localzirung der Erregungserscheinun- 
gen im Centralnervensystem (letter to the editor received 
Jan. 17, 1891). Cenlralbl. Physiol. 4:649, 1891. 

295. Caton, R. Die Strome des Centralnervensystems (letter 
to the editor received Feb. 22, 1891). Cenlralbl. Physiol. 4: 
785, 189!. 

296. ScHAFER, E. .\. Ti'xlhook of Physiology. Edinburgh: Young 
& Pentland, London: Morrison & Gibb; 1898, 1900. 2 vol. 

297. Beck, A. and Napoleon Cvbulskl VVeitere L'ntersuchun- 
gen iiber die elektrischen Erscheinungen in der Hirnrinde 
der .AflTen und Hunde. Cenlralbl. Physiol. 6:1, 1892. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



51 








ll..-w,.,,l, ,,„,, IX 




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p|«klr»<U im "lH/.;ip/ 


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I 


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•|Mi (intitiiiu ilra7in.-nia; i*.'t. ;U). 2."», in. -i"J. l.'t. lO; 
.irariiirnir ndii'ip [irzrdnifj 70; 
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■| r^/i 


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i'fy 


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u 


(K) ziiilra/nieniii iwiatJcni . I>2; 

l><i uilaiiin draznun-a; IV.. Il">. l.'iti. I.')*t, I'i7; 

lirzv (Int/niriiiii 'iJiii»;:i pr^'- Inirj : Ihii; 


. 


fij 1: 


|><i U'<taiiiii dra/nii-nia : 1 I't. 1 1" 



FIG. 37. Beck and protocol from one of the experiments in his original thesis on the electrical 
phenomena of the brain and spinal cord, i8go. (Obtained through the courtesy of Dr. Andrei Jus of 
Pruszkow.) 



Interest became widespread in 1929 with the first 
publication on brain potentials in man. In that year 
Hans Berger (298), a psychiatrist in a hospital in 
Jena, revealed to the scientific world the results of 
work he had been pursuing in secretive seclusion for 
over 5 years. He had repeated and confirmed the 
findings of Caton (to whom he gave full credit) and 
had extended them to man. He studied (and named) 
the electroencephalogram in normal man, finding 
the two major rhythms, alpha and beta, that Nemin- 
ski had found in dogs (299). He applied Caton's 
tests for the biological origin of the potentials he found, 
showing them to be affected by hypoxia and by anes- 
thesia. He also found them to be changed by sleep. 

Berger's outstanding contribution was the founda- 
tion of clinical electroencephalography. Having 
proved that brain waves could be recorded in man 
through the unopened skull, he went on to demon- 
strate that their characteristics could be used as an 
index of brain disease and thus he opened up a new 
line of approach for the physiologist and the clinician 
to the study of brain mechanisms. Berger's major dis- 
covery in the clinical field was that the electroen- 
cephalogram is abnormal in epilepsy. He did not with 

298. Berger, Hans (i 873-1 941). Uber das Elektrenkephalo- 
gramm des Menschen. Arch. Psychial. 87: 527, 1929. 

299. Prawditz-Neminski, W. W. Zur Kenntnis der elektrischen 
und der Innervationsvorgange in den functionellen 
Elementen und Geweben des tierischen Organismus. 
Elektrocerebrogramm der Saugcrtiere. Arch. ges. Physiol. 
209: 362, 1925. 




ii 

FIG. 38. Hans Berger, the first to record electroencephalo- 
graphic potentials from man, and the founder of clinical 
electroencephalography. Below is the first published electro- 
encephalogram of man. The subject was Berger's son, Klaus. 
His alpha rhythm is shown in the upper trace above a 10 per 
sec. sine wave from an oscillator. 

centainty record the spikes that are now associated 
with the seizure discharge, for with the technique he 
used there was serious interference by muscle poten- 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



tials. His instruments were a double-coil galvano- 
meter and a string galvanometer, and in much of his 
initial work he used only two electrodes, these being 
large plates fixed one to the forehead and one to the 
back of the head. He thus missed the localizing poten- 
tialities of the EEG, and in addition gathered in all 
the muscle potentials of the frontalis and trapezius 
muscles. In later experiments he changed to needle 
electrodes pushed into the skin. In his early experi- 
ments he tried a reference electrode consisting of a 
silver spoon held in the subject's mouth. The develop- 
ment of concepts about the EEG concomitants of 
grand mal epilepsy had their grounding in Fischer's 
(300) recordings during experimentally-induced 
seizures in dogs. 

The demonstration of the 3 per sec. wave-and- 
spike formation so typical of the petit mal type of 
epilepsy was the achievement of the team of Lennox, 
Davis and the Git)bses at the Harvard Medical .School 
(301). This discovery (which Berger came very close 
to making), together with that of Grey Walter (302) 
published the following year (1936), namely that 
brain tumors can be located through the skull by the 
abnormally slow waves of their surrounding tissue, 
form the two main foundations of clinical electro- 
encephalography. Altenburg & Foerster (303) had 
during a brain operation found abnormal potentials 
a.ssociated with a tumor, but Walter's demonstration 
that neoplasms could be located by the reversal of 
sign of the slow waves recorded from the unopened 
head and his confirmation that the tumor itself was 
electrically silent made this a practical clinical test. 
The subsequent expansion and development of 
electroencephalography is part of the continuing 
story of modern times not yet history. 

In the history of electroencephalography one other 
figure should be mentioned. One year after Caton's 
discovery, Danilewsky, the Russian neurophysiologist, 
noted the same phenomenon of oscillating cortical 
potentials in the absence of applied sensory stimula- 
tion in five dogs on which he was experimenting. He 
did not publish this at the time and reported it only 



in retrospect C304) as a confirmation of Caton's 
original ob.ser\ation. Danilewsky's primary interest 
lay in the autonomic eff'ects of stimulation of the cor- 
tex, such as arterial pressure changes (305), and in the 
mechanisms of temperature control (306), and he was 
active in the design of new instrumentation for electro- 
physiological experimentation (307). Together with 
his brother (Alexis Y. Danilewsky) he was prominent 
among the Russian physiologists at the end of the 
nineteenth century. 

In the latter half of the nineteenth century, Russian 
neurophysiology saw a development that was to in- 
fluence all future concepts about the brain and be- 
havior. At this period it was usual for Russian physiol- 
ogists to go to centers in Western Europe for training 
and experience under the outstanding teachers of the 
time, and to Miiller's laborator\' in 1856 came I. M. 
.Sechenov. Secheno\', later to be known as 'the father 
of Russian neurophysiologv' was then 27 years old and 
during the next 6 )ears he received training from six 
of the more outstanding physiologists: Miiller, du 
Bois-Reymond, Ludwig, \'on Helmholtz, Bunsen and 
Claude Bernard. The influence of these leaders can 
be traced in .Sechenov's later thought and develop- 
ment. Among them, only one, Miiller, retained even 
a lingering trace of allegiance to the concept of a vital 
force, and with him Sechenov- had the least contact, 
for Midler was at the end of his life, still lecturing but 
no longer experimenting. 

In neurophysiology the most influential of Seche- 
nov's teachers were du Bois-Reymond and Claude 
Bernard. Sechenov took du Bois-Reymond's cour.se in 
animal electricity and in i860 returned to St. Peters- 
burg with one of his master's induction coil stimula- 
tors and a galvanometer and with them introduced 
electrophysiology into Russian science. Two years later 
he was back in Western Europe, this time in Claude 
Bernard's laboratory in Paris, and it was here that 
the experiments were made that were to mold his 
thinking and to suggest to him a concept of brain 
mechanisms later to flower in the hands of Pavlov 
into the theor\' that has dominated Russian neuro- 



300. Fischer, Max H. Elektrobiologische ."^uswirkungen von 
Krampfgiften am Zentralnervensystem. Med. K/in. QMu- 
mch^'^g: 15, 1933. 

301. GiBBS, F. A., H. Davis and W. G. Lennox. The EEG in 
epilepsy and in conditions of impaired consciousness. 
A. M. A. Arch. .Neurol. & Psychial. 34: 1 133, 1935. 

302. Walter, VV. Grey. The location of cerebral tumours by 
electroencephalography. Lancet 2: 305, 1936. 

303. Foerster, O. and H. Altenburger. Elektrobiologische 
Vorgange an der menschlichen Hirnrinde. Deuliche 
Zlschr. .Nervenh. 135: 277, 1935. 



304. D.\nile\vskv, Vasili Y.\kovi.evich (1852- 1 939). Zur 
Frage iiber die elektromotorischen Vorgange im Gehirn 
als Ausdruck seines Thatig keitszustandes. Centralbl. 
Physiol. 5: I, 1 89 1. 

305. Danilewsky, V. Y. Experimentelle Beitrage zur Physi- 
ologic des Gehirns. Arch. ges. Physiol. 11 : 128, 1875. 

306. Danilewsky, V. Y. Die Verbrennungswarme der Nah- 
rungsmittel. Biol, ^enlralbl. 2: 371, 1882. 

307. D.^NiLEWsKv, V. Y. A new electrical machine for rhyth- 
mically altering the strength of galvanic currents (in 
Russian). Vralsch. 22, 1883. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



53 





FIG. 39. Ivan Michailovich Sechenov and his diagram illustrating reflex arcs in the spinal cord 
and brain of the frog, a-b-c-d represents a spinal refle.x arc with sensory (a-A), central (b-c) and 
motor ((■-(/) components. The reflex arc of the brain consists of the sensory nerve (O), the central 
component (.V-f) and the motor efferent (<:-(/). P is the region in the brain stem where Sechenov 
concluded the inhibitory apparatus lay. 



physiology ever since, the theory of conditional re- 
flexes. 

Sechenov's experiments that proved so crucial to 
his future thinking were on the effect on reflex move- 
ments of salt crystals placed at various levels of the 
transected neuraxis (308). His preparation (309) was 
the decapitated frog, a toe of which he dipped into 
acid, a procedure that had been developed by Tiirck. 
He timed the interval between stimulus and onset of 
withdrawal of the frog's foot by counting the beats of 
a metronome. In this way he got some index of the 
degree to which application of the salt crystal to the 
brain stem slowed withdrawal. Sechenov interpreted 
lengthening of withdrawal time as inhibition of reflex 
activity. The selection of a salt crystal as a stimulus 
seems strange in the hands of a pupil of du Bois- 
Reymond's and is reminiscent of Marshall Hall's use 
of it half a century earlier to study depression and 

308. Sechenov, Ivan Mich.mlovich (1829-1905). Physiolo- 
gische Studien iiber die Hemmungsmechanismus fur die Re- 
flexthdiigkeit des Riickenmarks im Gehirne des Frosches. Berlin : 
Hirschwald, 1863. 

309. Sechenov, I. M. Note sur les moderateurs des mouve- 
ments reflexes dans le cerveau de la grenouille. Acad. Sc, 
Paris 1863. 



augmentation of spinal reflexes. Only later (310) did 
.Sechenov use electrical stimulation in his experiments 
on the 'spontaneous' variations of spinal cord poten- 
tials which he regarded as signs of activity in the 
spinal centers. This was the first experimental ap- 
proach towards a centrally exerted inhibitory action 
on skeletal ('voluntary') muscle. 

Although at this stage his own experimental evi- 
dence seemed slender, Sechenov must have been 
pondering its meaning in much wider terms, for a 
year later, on his return to Rus.sia, he published as a 
series of articles the essay (31 1) that proved to be so 
influential in Rus.sian physiology. This essay on the 
Reflexes of the Brain was later (1866) published as a 
book after a stormy period during which efforts were 
made to suppress its publication and censure its 
author. This opposition was stirred by Sechenov's 
assertion that all higher brain function was a material 
reflex consisting of three sectors — an afferent initia- 
tion by sensory inflow, a central process entirely sub- 

310. Sechenov, I. M. Galvanische Erscheinungen an dem 
Verlangerten Marke des Frosches. Arch. ges. Physiol. 27: 
524, 1882. 

311. Sechenov, I. M. Reflexes of Ike Brain. Medizinsky Veslnik, 
1863; English translation in Sechenov's Selected Works. 
Moscow-Leningrad: State Publ. House, 1935, p. 263. 



54 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



ject to physical laws and an efferent component result- 
ing in a muscular movement. All reactions, however 
they might be described in common parlance as 
pleasure, fear, distress or other descriptive terms were, 
according to him, in essence muscular in expression. 
During the passage of the inflow through the central 
portion of the arc there could either be excitation 
which would augment the reflex motor response (as 
in so-called emotional states) or inhibition which 
would decrease the reflex muscular movement, the 
resultant beine; 'rational' controlled behavior. It is 
interesting that Sechenov conceived that inhibition 
could be learned and that with maturity an increase 
in the degree of inhibition exerted was achieved. 

Thus, according to Sechenov, all human behavior 
was a balance between inhibition and excitation 
operating mechanically at the central link of the re- 
flex arc. A so-called 'willed' movement according to 
him only apparently lacked the first component of the 
arc, its afferent inflow being material memory traces 
left by external stimuli in the past. It was in elaborat- 
ing this part of his theory that Sechenov approached 
the concept of the conditional reflex, for he postulated 
that the memory trace of a past sensory experience 
could be evoked by the recurrence of any fraction of 
it even if this fraction were quite insignificant and 
unrelated in its apparent meaning. This is essentially 
the principle underlying the formulation of the condi- 
tional reflex theory, namely the potency of an indiffer- 
ent external stimulus provided it is repeatedly time- 
locked to the original experience. One further point 
should be noted in this early attempt to relate mental 
processes to brain physiology. Sechenov believed that 
man had the special faculty of increasing the degree 
of inhibition exerted at the central link until a level 
of total inhibition of the efferent discharge was 
reached, and he held that thought was an example of 
this condition. 

Although terms such as 'cerebral reflexes' and 
'psychical reflexes' abound in the nineteenth century 
literature, they were mostly used by psychologists 
to describe automatisms. At this period only a few 
writers had broached the problem of explaining 
mental processes in physiological terms. Thomas Lay- 
cock C312), whose belief in cortical localization no 
doubt influenced his pupil Hughlings Jackson, wrote 
in 1845 a paper On the reflex function of the brain. In this 
he stated his belief that "the brain although the organ 
of consciousness, was subject to the laws of reflex ac- 
tion, and that in this respect it did not differ from other 

312. Laycock, Thomas (1812-1876). On the reflex function 
of the brain. Brit. & For. Med. Rev. 19: 298, 1845. 



ganglia of the nervous system." He too envisaged a 
three-component arc, the central link in the brain 
being one where 'ideagenous' changes took place that 
influenced the motor output. He came close to antic- 
ipating one of Sechenov's postulates by stating that 
the actual sensory impression of an object or the mere 
idea of it could evoke the same 'ideagenous' change 
in the brain and result in a similar reflex motor effect. 
So firmly did Laycock believe in the neuronal basis 
of ideas that he calculated how many there could be 
to the square inch of grey matter (the answer was 
8000) and argued that "as there must be an immense 
number of square inches of surface in the grey matter 
extended through the cerebrospinal axis of man, there 
is space sufficient for millions." We find echoes of this 
kind of calculation in some of today's conjectures 
about the number of possible interconnections in the 
brain. 

Laycock did not test his hypotheses by experiment 
though he argued from a basis of clinical observation, 
for he said "an experiment is occasionally made by 
nature." There is no evidence that Sechenov was 
aware of Laycock's ideas, although he was influenced 
by the writings of two other nonexperimentalists, 
Herbert Spencer (313) and George Henry Lewes 
(314). These two men, united through their relation- 
ships with George Eliot, were influential not only on 
Sechenov but on Pavlov. Their writings, now largely 
unread, were translated into Russian almost immedi- 
ately after publication and were everywhere highly 
regarded. .Spencer's work was an argument for cortical 
representation of mental function, and Hughlings 
Jackson was one who expressed indebtedness to him. 
Spencer based much of his argument on comparative 
evolution though he was writing 4 years before the 
publication of the Origin of the Species by Darwin 
(315), another writer whose books were extremely 
influential on Ru.ssian thought. Spencer stressed 
localization of mental processes, saying that "whoever 
calmly considers the question cannot long resist the 
conviction that different parts of the brain must in 
some way or other suhserve different kinds of mental 
action." When we find in his Autobiography (316) that 

313. Spencer, Herbert (1820-1903). Principles of Psychology. 
1855. 2 vol. 

314. Lewes, George Henry (18 17-1 878). The Physiology of the 
Common Life. London: Blackwood, 1859. 2 \'ol. 

315. Darwin, Charles Robert (1809-1882). On the Origin of 
Species by means of .Kaiural Selection or the Preservation of 
Favoured Races in the Struggle for Life. London: John 
Murray, 1859. 

316. Spencer, H. .In Autobiography. London: Williams & 
Norgate, 1904. 2 vol. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 




FIG. 40. Ivan Petrovich Pavlov (reproduced from Babkin's Pavlov, A &'ogra/)A> by permission of the 
publishers. University of Chicago Press). On the right Pavlov watching an experiment (Sovfoto 
* 3' 9573 Moscow USSR). 



he had the bumps on his head read by a phrenologist 
(with flattering interpretations)^' we perceive a deri- 
vation of his ideas from Gall and Spurzheim. Spencer 
became hypochondriacal about his own head, believing 
it to have an inadequate blood supply. To improve 
the circulation he exercised at rowing and at racquets 
in 15 mill, spurts, dictating his books in the intervals 
between exertions. His friend, Lewes (3 1 7) in his 
Physical Basis of Mind was doubtful about the localiza- 
tion of the various mental processes but convinced of 
their physiological nature. 

Pavlov, the towering figure of Russian neurophysiol- 
ogy, repeatedly throughout his life stressed his in- 
debtedness to Sechenov'" and to Lewes^' (whose book 
on physiology he read when a schoolboy). The 
influence of these men, one too little known outside 
Russia, one almost forgotten, was so great that they 
feature not only in the scientific writings of the times 
but in Russian fiction. Turgeniev is said to have 
taken Sechenov as his model for the science student, 
Bazarov, in Fathers and Sons and Dostoievsky cited 
the reading of Lewes' book as a sign of education in 
the wife of a drunk in Crime and Punishment. 

Pavlov dated his interest in the digestive system 
(318) from reading Lewes, an interest that was to 
occupy the first 25 years of his working life and to win 
for him the Nobel Prize. And it was a feature of the 
digestive system, the salivary apparatus, that was to 
be drawn by him into the work suggested by Sech- 

317. Lev^^s, G. H. The Physical Basis of Mind. Boston, 1877. 

318. Pavlov, I. P. (1B49-1936). Lectures on the Work of the 
Principal Digestive Glands (in Russian). St. Petersburg: 
Kushnerev, 1897; translated into English by W. H. 
Thompson. London: Griffin, 190J. 



enov's theories of 30 years before. Fundamental in 
Pavlov's thinking (319) was the concept of temporary 
connections established in the cortex by the repetition 
of external stimuli linked only by a constant time 
interval, although one gets the impression that he 
thought more in terms of influence than of specific 
neuronal connections. Thus, for example, in his 
classical experiment, the repeated sound of a metro- 
nome, at a fixed interval before food was made avail- 
able to his experimental dogs, caused salivation to 
begin with shorter and shorter latency and at an 
increasing rate. Later more complex situations were 
developed as laboratory procedures, and this type of 
reflex was used for mapping the response of the 
cerebral cortex to various sensory inputs, Pavlov 
(319) naming the areas as "analyzers' for the various 
modalities. 

The instability and temporary character of the 
conditioned reflex in contrast to that of the inborn 

319. Pavlov, I. P. Lectures on Conditioned Reflexes, English 
translation by \V. H. Gantt. New York: Internat. Pub., 
1928. 



"'The opening sentence of the phrenologist's report read: 
"Such a head ought to be in the Church." When we seek the 
basis for this statement in the itemized score for Spencer's 
bumps, we find both Firmness and Self-esteem 'very large;' 
Language 'rather full,' and Wit and Amativeness only 'moder- 
ate.' 

'° See Shaternikov, M. N. The life of I. M. Sechenov. In : 
Sechenov, Selected Works. Moscow-Leningrad: State Publ. House, 

■935- 

^' See Babkin, B. P. Pavlov. Chicago: Univ. Chicago Press, 
1949, p. 214. 



56 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



unconditioned reflexes serving instinctual movements 
for preservation of life led to Pavlov's ideas of cortical 
inhibition and its relationship to sleep and hypnosis. 
Pavlov distinguished between natural conditional 
reflexes learned in early life and the artificially 
conditional reflexes of the laboratory. Among the 
first he classed the connections formed in infancy 
between the smell or sight of food and the salivary 
response. This observation goes back many centuries 
and is well described by Whytt (i8i), who like Lay- 
cock after him, recognized that the 'idea' could be as 
powerful a stimulus as the sensory impression. 

Pavlov had in his youth been a student of Ludwig 
and of Heidenhain; from the former he had brought 
the insistence on a physical basis for all biological 
processes and from the latter an interest in secretory 
mechanisms and the phenomena of hypnosis. The 
fertility of Pavlov's ideas and his indefatigable energy 
drew to him an enthusiastic school of workers and b\' 
the 1920's he had a large team working under him on 
the many features of establishment, reinforcement, 
extinction and inhibition of conditional reflexes. He 
was a well-loved teacher, though a man of fiery 
temperament. Sherrington has left a vignette of him 
at the age of 65 describing him as "overflowing with 
energy, although an elderly man; he was spare in 
figure and alert and humourous in manner." Even 
at the end of a long working day on encountering a 
stairway he "ran up it rather than walked." Sherring- 
ton came away from this visit, made in 191 4, with a 
great enthusiasm for the leader of Russian neuro- 
physiology C320). 

Pavlov's ideas of the reflex became more diffuse 
and more nebulous as he grew older. Experiments to 
test the modes of behavior of animals to conditioning 
stimuli were less difficult to design than ones to test 
the hypothesis advanced to explain them. Temporary 
neuronal connections in the cortex proved easier to 
postulate than to prove. Pavlov's own attempts were 
with decorticate preparations (a technique that had 
been u.sed before him by Sechenov) and it is only in 
recent times that the electrophysiologist's tools have 
been applied to this problem. 

320. Sherrington, C. S. Marginalia. In: Science, Medicine and 
History. Essays in Honour of C. Singer, edited by E. A. Un- 
derwood. London: Oxford, 1953. 



As the second half of the twentieth century unfolds 
the neurophysiologist in his search for brain mech- 
anisms continues to use the three main categories of 
experimental procedure: anatomical, ablative and 
electrical. It is the great advance in electrical stimula- 
tion and recording that marks this era of investigation 
from its predecessors, although it is only through 
knowledge from all sources that progress can be 
achieved in an understanding of the brain. 

Neurophysiology came into being as a specialized 
branch of endeavor when the nervous system no 
longer had to compete with the humors and with the 
blood as the principal coordinator of the body. With 
the recognition that sensation and motion were medi- 
ated by the nerves their position became unassailable, 
for movement was regarded as the sign of life. Slowly 
the concept of neural organization began to be pieced 
together and levels of integration were postulated, in 
the spinal cord, in the cortex and in the deeper struc- 
tures of the brain. The period of analysis of the func- 
tion of each structural unit, of each sector of the 
nervous system, was followed b\' a shift of emphasis 
towards a synthetic consideration of neural activity. 
The search began for the physiological mechanisms 
of mental processes, of consciousness, of memory — all 
terms and concepts that had belonged to another do- 
main of thought. In the neurophysiolog\' of today we 
find both angles of approach, ranging from analysis 
of the intimate physicochemicai basis of nervous 
structure and dynamics to the synthesis of action that 
we call behavior of the organism. 

The writer expresses her great indebtedness to the authors 
of many articles and books not listed in the abridged bibli- 
ography that follows. She adds her thanks to those who have 
sent her material in correspondence, and in particular would 
mention appreciatively: Dr. Maria Rooseboom for the use of 
material and microfilms from the National Museum for the 
History of .Science at Leiden; Dr. Palle Birkelund, Director of 
the Danish Royal Library; Dr. .\uguste Tournay for a photo- 
stat copy of Pourfour du Petit's Letters, the Institution of Elec- 
trical Engineers and Miss Helen G. Thompson for access to 
material collected by Silvanus P. Thompson on Gilberd; Miss 
Anne Caton for family photographs and material from the 
diaries of Richard Caton; Dr. Andrei Jus of Pruszkov for photo- 
stats of Adolf Beck's doctoral thesis; and F. Czubalski of War- 
saw for information about Beck's works. For details of Beck's 
life the writer expresses warm appreciation to his daughter, 
Mme. Jadwiga Zahrzewska. 



A SHORT LIST OF SECONDARY SOURCES 

Space does not permit the listing of all the articles to whose 
authors the writer is indebted for information. The following 
books have been selected for the special interest they may 
have for the physiologist. Where possible, works in the English 
language have been chosen. 



Bence Jones, H. On Animal Electricity. London: Churchill, 1852. 
Bettmann, O. L. .4 Pictorial History of .Medicine. Springfield: 

Thomas, 1956. 
Boring, E. G. A History of Experimental Psychology. New \'ork: 

.\ppleton, 1 929. 



THE HISTORICAL DEVELOPMENT OF NEUROPHYSIOLOGY 



57 



BosTOCK, J. Sketch of the Hutory nf Medicine from its Origin to 

the Commencement of the .\ineteenlh Century. London: Sherwood, 

Gilbert and Piper, 1835. 
Brown, G. B. Science, its Method and 1/1 Philosophy. London: 

Allen & Unwin, 1951 
Canguilhem, G. La Formation du Concept de Reflexe. Paris: 

Presses univ. France, 1955. 
Castiglioni, a. Italian Medicine. New York: Hoeber, 1932, 

Clio Medica Series, vol. 6. 
Castiglioni, A. .-i History of Medicine (2nd ed.)- New York: 

Knopf, 1947. 
CoMRIE, J. D. A History of Scottish Medicine. London: Wellcome 

Historical Medical Museum, 1932. 
Corner, G. \V. Anatomy. New York : Hoeber, 1 930, Clio Medica 

Series, vol. 3. 
Cooke, J. A Treatise on Nervous Diseases. Boston: Wells and 

Lilly, 1824. (Previously published in England.) 
Dampier, W. C. a History of Science. Cambridge: Cambridge, 

1946. 
Dana, C. Textbook of Nervous Diseases. New York : Wm. Wood, 

1925. (Includes a chapter by F. H. Garrison on the history 

of neurology.) 
D.^REMBERG, C. Essai sur la determination et ies caracthes des 

periodes de I'histoire de la medecine. Gaz. med. Paris, 1850. 
Fearing, F. Reflex Action. Baltimore: Williams & Wilkins, 1930. 
Foster, M. Textbook of Physiology (1st American ed.), edited 

by E. T. Reichert. Philadelphia: H. C. Lea's son and Co., 

1880. (1st English ed., 1876.) 
Foster, M. Lectures on the History of Physiology. Cambridge: 

Cambridge, igoi. 
Fr.^nki.in, K. a Short History of Physiology. London: Staples, 

1949- 
Fulton, J. F. Muscular Contraction and the Reflex Control of 

Movement. Baltimore: Williams & Wilkins, 1926. 

Fulton, J. F. Selected Readings in the History of Physiology. Spring- 
field : Thomas, 1930. 

Fulton, J. F. Physiology. New York: Hoeber, 1931, Clio Medica 
Series, vol. 5. 

Fulton, J. F. Physiology of the Nervous System (3rd ed.). New 
York : O.xford, 1 949. 

Garrison, F. H. An Introduction to the History of Medicine. Phila- 
delphia : Saunders, 1929. 

Hall, A. R. The Scientific Revolution ijou-iSuo. London: 
Longmans, 1954. 

Hamilton, W. The History of Medicine, Surgery and Anatomy from 
the Creation of the World, to the Commencement of the Nineteenth 
Century. London: Colburn and Bentley, 1831. 

Lenard, p. Great Men of Science, translated by H. S. Hatfield. 
New York: Macmillan, 1933. 

M.«.jor, R. --1 History of Medicine. Springfield: Thomas, 1954. 

Morgan, C. E. Electro-Physiology and Therapeutics. New York: 
Wood, 1868. 



Morton, L. T. and F. H. Garrison. A Medical Bibliography 

(2nd ed.). London: Grafton, 1954. 
Nordenskiold, E. History of Biology, translated by L. B. Eyre. 

New York: Tudor, 1935. 
Pettigrev\', T. J. Medical Portrait Gallery. Biographical Memoirs 

of the most Celebrated Physicians, Surgeons, etc. London: Fisher, 

1872. 3 vol. 
PoTAMiN (Brother) and J. J. Walsh. Makers of Electricity. 

New York: Fordham Univ. Press, 1909. 
Renouard, p. V. History of Medicine from its Origin to the .\ine- 

teenth Century, translated by C. G. Comegys. Philadelphia: 

Lippincott, 1856. 2 vol. 

Rothschuh, K. E. Geschichte der Physiologic. Berlin : Springer, 

1953 
Russell, T. R. The History of Heroes of the Art of Medicine. 

London: Murray, 1861. 
ScHAFER, E. A. Textbook of Physiology. Edinburgh and London: 

Y. J. Pentland, 1898, vol. i ; 1900, vol. 2. 
Shryock, R. H. The Development of Modern Medicine. New 

York: Knopf, 1947. 
Singer, C. .-1 Short History of Medicine. New Y'ork: Oxford, 1928. 
Singer, C. J. Essays on the History of Medicine. London: Oxford, 

■9^4- 
Singer, C. J. The Evolution of Anatomy; a Short History of Ana- 
tomical and Physiological Discovery to Harvey. London : Paul, 

Trench, Trubner, 1925. 
Soury, J. Le Systhne nerveux central. .Structure el Fonctions. Paris: 

Carre et Naud, 1899. 
Sprengel, K. Histoire de la Medecine, translated and abridged 

from the 2nd German ed. by A. J. L. Jourdan. Paris: 

Deterville, 1792-1803. 2 vol. 
Stirling, W. Some Apostles of Physiology. London: Waterlow, 

1902. 
Sudhoff, K. Essays in the History of Medicine, English translation 

edited by F. H. Garrison. New York : Medical Life Press, 

1926. 
Whewell, W. History of the Inductive Sciences. London: Parker 

1837- 3 ™1- 

Whittaker, E. T. History of the Theories of Aether and Elec- 
tricity from the Age of Descartes to the Close of the Nineteenth 
Century. London and New York: Longmans, Green and Co., 
igio, revi.sed 1951, second volume added 1953. 

Wightman, W. p. D. The Growth of Scientific Ideas. New Haven: 
Yale Univ. Press, 1951. 

Wilkinson, C. H. Elements of Galvanism in Theory and Practice. 
London: Murray, 1804. 

Wolf, A. A History of Science, Technology and Philosophy in the 
1 6th and lyth Centuries (2nd ed.). London: .-\llen and L'nwin, 

I950- 
Wolf, A. A History of Science, Technology and Philosophy in the 
1 8th Century (2nd ed.). London: .\llen and Unwin, 1952. 



BIOGRAPHIES 

For each of the following scientists one biographical study 
only has been listed. Again the choice has been made on the 
grounds of interest to the physiologist and, where possible, text 
in the English language. 

Aristotle. Taylor, A. E. Aristotle. London: Nelson, 1943. 
Bacon, Francis (i 561-1626). Farrington, B. Francis Bacon, 
Philosopher of Industrial Science. New York: Schuman, 1949. 



Baglivi, Giorgio (1668-1707). Stenn, F. Giorgio Baglivi. Ann. 

Med. Hist. (3rd ser.) 3: 183, 1941. 
Bell, Charles (i 774-1 842). Pichot, A. The Life and Labours of 

Sir Charles Bell. London: Bentley, 1880. 
Berger, Hans (1873-1942). Ginzberg, R. Three years with 

Hans Berger. .\ contribution to his biography. J. Hist. Med. 

4: 361, 1949- 
Bernard, Cl.aude (1813-1878). Olmsted, J. M. D. and E. H. 



58 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



Olmsted. Claude Bernard, Physiologist, and the Experimental 

Method in Medicine. New York: Schuman, 1952. 
BicHAT, Marie Francois Xavier (1771-1802). Busquet, P. 

Les Biographies Medicates I: 37, 1927. 
BoERHAAVE, HERMANN (1668-1738). Burton, W. An Account of 

the Life and Writings of Hermann Boerhaave. London: Lintot, 

1743- 
CoTUGNO, DoMENico (1736-1822). LevinsoH, A. Domenico 

Cotugno. Ann. Med. Hist. 8: i, 1936. 
DA Vinci, Leonardo (1452-1519). O'Malley, C. D. and J. 

B. deC. M. Saunders. Leonardo da Vinci on the Human Body. 

New York: Schuman, 1952. 
Descartes, Rene (1596- 1650). Haldane, E. The Life of Rene 

Descartes. London, 1905. 
Fernel, Jean (1497-1558). Sherrington, C. S. The Endeavour 

of Jean Fernel. Cambridge: Cambridge, 1946. 
FoNTANA, Felice (1730-1805). Marchand, J. F. and H. E. 

Hoff. Felice Fontana. The Laws of Irritability. J. Hist. Med. 

10: 197, 302, 399, 1955. 
Galen (130-200). Sarton, G. Galen of Pergamon. Lawrence: 

Lfniv. Kansas Press, 1954. 
Gall, Franz Joseph (1758-1828). Temkin, O. Gall and the 

phrenological movement. Bull. Hist. Med. 21 : 275, 1947. 
Galvani, Alovsius (i 737-1 798). Fulton, J. F. and H. Gush- 
ing. A biographical sketch of the Galvani and Aldini writings 

on animal electricity. Ann. Sci. i : 239, 1936. 
GiLBERD, William (1540 or 1544- 1603). Waldron, F. G. 

Biographical Mirrour. London : Harding, 1 795. 
Hales, Stephen (1677-1761). Burget, G. E. Stephen Hales, 

1677-1761. Ann. Med. Hist. 7: 109, 1925. 
Hall, Marshall (1790- 1857). Hall, Charlotte. Memoirs of 

Marshall Hall. London, 1861. 
Harvey, William (1578-1657). Chauvois, L. William Harvey. 

His Life and Times.- his Discoveries: his Methods. London: 

Hutchinson, 1957. 
Horslev, Victor Alexander Haden (1857-1916). Paget, S. 

Sir Victor Horsley, a Study of his Life and Work. New York: 

Harcourt Brace, 1920. 
Hunter, John (i 728-1 793). Paget, S. John Hunter. London: 

Fisher Unwin, 1897. 
LiNACRE, Thomas (1460-1524). Johnson, J. N. The Life of 

Thomas Linacre. London, 1835. 
LuDWiG, Carl Friedrich Wilhelm (i 816-1 895). Lombard, 

W. P. The Life and Work of Carl Ludwig. Science 44: 363, 

1916. 
M.agendie, Francois (1783-1855). Olmsted, J. M. D. Francois 

Magendie. New York: Schuman, 1944. 
Monro, .^LEXANDER (1697-1762) and Monro, .Alexander 



Secundus (1733-1817). Inglis, J. A. The Monros of Auchin- 
bowie. Edinburgh, 1911. 

MOller, Johannes (1801-1858). Haberling, W. Johannes .Mai- 
ler. Leipzig, 1924. 

Nollet, Jean Antoine. Torlais, J. Un Physicien au Steele des 
Lumieres: Abbe JVollet. Paris: Siprico, 1954. 

Oersted, Hans Christian (i 770-1851). Stauflfer, R. C. 
Speculation and experiment in the background of Oersted's 
discovery of electromagnetism. Isis 48: 33, 1957. 

Pavlov, Ivan Petrovich (1849-1936). Babkin, B. P. Pavlov. 
Chicago: Univ. Chicago Press, 1949. 

Prochaska, Jiri (1749-1820). Laycock, T. Introduction. 
To: The Principles of Physiology. London: Sydenham .Society, 
1851, p. ix. 

Ramon y Cajal, Santiago (1852-1934). Cannon, D. Explorer 
of the Human Brain. New York: Schuman, 1949. 

Sechenov, Ivan Mihailovich (1829-1905). Shaternikov, M. 
N. The life of I. M. Sechenov (in English). In: Sechenov, 
Selected Works. Moscow-Leningrad: State Publ. House, 1935. 

Stahl, Georg Ernst (1660-1734). Metzger, H. Newton, Stahl, 
Boerhaave et la doctrine chimique. Paris: Alcan, 1930. 

Stensen, Nicholas (1638-1686). .Nicolaus Steno and His Indice, 
edited by G. Scherz. Copenhagen: Munksgaard, 1958. 

Unzer, Johann August (1727-1799). Laycock, T. Introduc- 
tion. To: The Principles of Physiology. London: Sydenham 
Society, 1851, p. i. 

VAN Leeuwenhoek, .Antonj (1672-1723). Dobell, C. Antony 
van Leeuwenhoek and his "Little Animals." New York: Harcourt 
Brace, 1932. 

Vesalius, .■\ndreas (1514-1564). Gushing, H. A Bio-bibliog- 
raphy of Andreas Vesalius. New York: .Schuman, 1943. 

Volta, Alessandro (1745-1827). Cohen, I. B. Introduction. 
To: Galvani's Commentary, English translation by M. G. 
Foley. Norwalk: Burndy Library, 1954. 

von Guericke, Otto (1602-1686). Hoffmann, F. W. Otto von 
Guericke. Magdeburg, 1874. 

VON Haller, Albrecht (1708-1777). Klotz, O. Albrecht von 
Haller 1708-1777. Ann. .Med. Hist. 8: 10, 1936. Also: Hem- 
meter, J. C. Albrecht von Haller, his scientific, literary and 
poetic activity. Bull. Johns Hopkins Hosp. 19: 65, 1908. 

VON Helmholtz, Hermann Ludvvig Ferdinand (1821-1894). 
McKendrick, ]. G. H. L. F. von Helmholtz- London: Unwin, 

1899- 
VON Humboldt, Frederick .'\lexander (i 769-1859). de Terra, 

H. The Life and Times of Alexander von Humboldt. New York: 

Knopf, 1955. 
Whytt, Robert (1714-1766). Seller, W. Memoir of the Life and 

Writings of Robert Whytt, .\L D. Edinburgh: Neill, 1862. 
Willis, Thomas (1621-1675). Miller, W. S. Thomas Willis 

(1621-1675). Bull. Soc. Med. Hist. Chicago 3: 215, 1923. 



CHAPTER II 



Neuron physiology — introduction 



J. C. ECCLES I Department of Physiology^ Australian National University, Canberra, Australia 



CHAPTER CONTENTS 

Morphological Features of the Neuron 

Physiological Properties of Surface Membranes of Neurons 

Transmission Between Neurons 

Excitatory Synaptic Action 

Inhibitory Synaptic Action 

Factors Controlling Impulse Generation 

Central Inhibitory Pathways 

Inhibitory and Excitatory Transmitter Substances 



MORPHOLOGICAL FEATURES OF THE NEURON 

THE CONCEPT that the nervous system is composed of 
discrete units or nerve cells was first proposed in 
1886-7 tiy His and Forel, later it was strongly sup- 
ported by van Gehuchten and Cajal, and finally in 
1 89 1 it was given an appropriate nomenclature, 
' neuron' and ' neuron-theory', by Waldeyer. Al- 
though all the great neurohistologists of that classical 
era were ranged for or against the neuron theory, it 
was pre-eminently the achievement of Cajal to estab- 
lish the fact that the functional connections between 
individual nerve cells, or neurons, are effected by 
close contacts and not by continuity in a syncytial 
network, as was proposed in the rival reticular theory 
of Gerlach and Golgi. Appropriately, Cajal's last 
great contribution (11) was devoted to a critical sur- 
vey of the evidence for and against the neuron theory, 
which has not been seriously challenged since that 
time, at least for the vertebrate nervous system. 

Neurons have the most diverse forms, yet there are 
certain features that are common to all. The nucleus 
always lies in an expanded part, the soma or cell 
body, from which the axon takes origin and often runs 
for long distances before breaking up into the synaptic 



terminals that make contact either with other neurons 
or with effector cells such as muscles, glands or elec- 
tric organs. Under physiological conditions of opera- 
tion, axons (with the exception of primary afferent 
axons) transmit impulses only in the centrifugal 
direction and thus constitute the effector apparatus of 
the nerve cell. The different types of nerve cells show 
much more variation in their other branches, the 
dendrites, which normally share with the soma the 
receptive function for the nerve cell. Pyramidal cells 
of the cerebral cortex and the Purkinje cells of the 
cerebellum have the most extensively branched den- 
drites, but most neurons of the central nervous system 
have fairly elaborate dendritic structures. By contrast, 
in the dorsal root ganglion cells the receptive structure 
is remotely located in the receptor organs which are 
connected to the soma by a long axon-like fiber that 
normally conducts in the centripetal direction, and 
which we may call the primary afferent axon. 

\'ery great functional significance is attached to the 
surface membrane of the neuron. This membrane 
must not be confused with the fibrous, glial and 
myelin structures which contribute a sheath to 
neurons, providing them with mechanical strength 
and electrical insulation. Until the advent of electron- 
microscopy the surface membrane had not been ob- 
served directly; yet it was an essential postulate in 
explanations of the electrical properties of the surface 
of the neuron and of the manner in which its interior 
was maintained at a very different compo.sition from 
the exterior, particularly in respect to such ionic 
species as sodium, potassium and chloride. It also 
provided a structural basis for explaining such funda- 
mental processes as the conduction of the impulse and 
the operation of excitatory and inhibitory synaptic 
junctions. Recently, numerous electronmicroscopic 



59 



6o 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



Studies (21, 22, 2;5, 48, 66, 67, 70) have revealed it as a 
boundary membrane of uniform thickness, about 
50 A, which stands out with remarkable clarity from 
the interior of the neuron and its surround. There is 
much more uncertainty with respect to the chemical 
composition of the membrane, which generally is sup- 
posed to be a thin, proi)ai)ly bimolccular, layer of 
mixed phospholipids and cholesterol, supported by a 
protein framework. It is further postulated that the 
transport of molecules and ions across this membrane 
is largely a diffusion process, the respective net move- 
ments being determined by the electrochemical 
potentials However, metabolic energy must also be 
made available for net transport against the electro- 
chemical gradients of such ions as sodium and potas- 
sium. With some memljranes, it is also necessary to 
postulate that specific permeability functions are 
'built in'; for example, in all membranes giving the 
self-regenerative responses that are characteristic of 
impulses, depolarization initiates a brief permeability 
to sodium ions; and at excitatory synapses the excita- 
tory transmitter substance probably causes the mem- 
brane to become like a sieve with pores permeable to 
all small ions, while at inhibitory synapses the inhib- 
itory substance causes much more selective ion 
permeability, which may, however, be due to a still 
finer sieve-like structure. 

It will emerge in the sub.sequent .sections on neuron 
physiology that as yet very little functional significance 
can be attached to all the detailed structural features 
occurring within neurons, which are well described in 
a recent review by Young (79). At the present level of 
understanding, the behavior of neurons is explained in 
terms of the properties of their surface membranes, 
including the specialized surface membranes of the 
synaptic regions. The interior is assigned a function 
merely on account of its ionic composition and its 
specific conductance. Doubtless this unsatisfactory 
state of affairs will be remedied as new insights are 
gained into the metabolic functions of the nerve cell 
and their integration with the membrane functions. 

Some beginnings have already been made. For 
example, energy derived from metabolic processes in 
the neuron is necessary in order to move ions across 
the surface membrane against their electrochemical 
potentials. There is now evidence that, with the 
linked transfer of sodium outwards and potassium 
inwards, the rate of this ionic pump is determined by 
the internal concentration of sodium ions (15, 16, 53, 
54). Another correlation between the neuron interior 
and the surface membrane is beginning to emerge in 
relation to the synaptic vesicles in the presynaptic 



terminals. There is evidence .supporting the postulate- 
that these vesicles are concerned in the quantal 
emission of transmitter from the presynaptic terminals- 
of the neuromuscular junction (26, 63, 70); and that 
the level of the membrane potential of the presynaptic 
terminals determines the rate of emission of quanta 
therefrom, the rate rising by more than a million-fold 
during a nerve impulse. Thus it has been postulated 
that in some way the properties of the .surface mem- 
brane are able to influence profoundly the state of 
relatively large structures (spheres of 300 to 500 A in 
diameter) in the immediately adjacent cytoplasm 
(26, 63); and, by analogy, a similar postulate has been 
suggested for the synaptic vesicles which also form 
characteristic features of all synaptic junctions that on 
other grounds are regarded as functioning by chemical 
transmission (21, 29, 67). 

The internal structure of neurons is profoundly 
altered in pathological states induced, for example, by 
section of the axon or by the action of toxins (4). 
There is good evidence that such a striking feature as- 
the Nissl substance or ergastoplasm is concerned in 
the protein manufacture that occurs during growth 
and regeneration (58). But as yet there is little under- 
standing of the ' trophic' action which the cell body 
exercises on the axon, apparently by maintaining an 
intra-axonic pressure and a continual tran.sfcr of 
material along the fiber (79). 

Electronmicroscopy has already contributed much 
information that is of the greatest value in interpreting 
the mode of operation of synapses. Despite the very 
wide range in the grosser features of synapses, at the 
electronmicroscopic level there is a remarkable simi- 
larity between all synapses that are believed to work 
by a chemical transmitter mechanism (fig. i). Es.sen- 
tially, in these structures considerable areas of the 
presynaptic and postsynaptic membranes are sepa- 
rated bv a very narrow cleft that shows a remarkable 
uniformity in width for any one type of synap.se and 
that varies in width from 150 to 500 A with different 
types Presumably, this accurate apposition of the 
two membranes is maintained by some structural 
linkage across the cleft, which appears in elcctron- 
microphotographs as a granular material The pre- 
synaptic and postsynaptic membranes are continuous 
with the surface membranes of their respecti\e cells, 
neurons or effector cells, and as yet ha\e not been 
shown to have any distinctive structural features 
except the deep transverse folds that distinguish the 
subsynaptic mu.scle membrane at the neuromuscular 
junction (figs. iZ), E) (19, 69, 70). Finally, in all 
cheinical-transmitting synapses the presynaptic termi- 



NEURON PHYSIOLOGY INTRODUCTION 6 1 




FIG. I . Drawings showing dimensions and form of various types of synaptic junctions as revealed 
by electronmicroscopy. In all transverse sections the presynaptic terminals are sho%vn above and the 
postsynaptic element below. In addition the presynaptic terminals can be identified by the contained 
synaptic vesicles. The synaptic cleft is seen as the narrow space between the juxtaposed presynaptic 
and subsynaptic membranes and is shown communicating at the sides of the synapse with the inter- 
stitial spaces. A, A large synapse on a motoneuron of the abducens nucleus. [From Palay (67).] B. 
Synapse in the ventral acoustic ganglion of the guinea pig. [From de Robertis (21).] C. Synapse 
between red receptor and postsynaptic cell in the rabbit retina. [From de Robertis & Franchi (23).] 
D, E. Elongated nerve terminal of amphibian muscle as seen from above (Z)) and in transverse 
section (£). The naturally occurring irregularities of the junctional folds are neglected in order to 
give a regular geometrical diagram with approximately equivalent dimensions. A junctional fold is 
shown by a broken line in E. [From data and figures of Couteaux & Taxi (19) and Robertson 
(70).] 



nals contain the characteristic synaptic vesicles which 

o 

are 300 to 500 A across and which are often ckistered 
close to the synaptic region. 

The word synapse, as proposed by Sherrington 
(71), may be applied to the presynaptic terminal with 
its contained synaptic vesicles, the synaptic cleft of 
150 to 500 A, and the subsynaptic membrane with its 
special receptive and reactive mechanism. Later, 
when the mode of operation of synapses is discussed, 
it will appear that much of the old morphological 
characterization of synaptic endings is of little signifi- 
cance, at least for many types of neurons. Thus the 
various localizations designated axosomatic, axoden- 
dritic and axoaxonic would be almost equipotent in 
their action except for those neurons that have very 
elongated dendrites, as for example the pyramidal 
cells of the cortex. Furthermore, there can be little 



significance in the detailed form of synapses as de- 
scribed bv such terms as hautons lerminaux and en 
passant, giant club endings, basket-type endings, etc. 
[cf. Bodian (3)]. 



PHYSIOLOGICAL PROPERTIES OF SURF.iiCE 
MEMBRANES OF NEURONS 

By inserting an electrode within a nerve fiber or 
the soma of a neuron and analyzing the potential 
changes produced by current pulses, it has been 
shown that the surface membrane has a high electrical 
resistance, corresponding to its low ionic permeability, 
and a high electrical capacity, as would be expected 
for a membrane no more than 50 A thick. The elec- 
trical resistance shows wide variations with different 



62 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



types of nerve fibers and neurons, the values ranging 
from looo to approximately io,ooon-cm- for squid 
and sepia giant fibers, respectively (49, 77), and it 
probably lies within the range of 500 to loooli-cm^ 
for mammalian motoneurons (15, 29, 44). Values for 
specific capacitance of giant fiber membranes range 
from I to 1.5 mF per cm- and for mammalian moto- 
neurons are probably at least 3 nF per cm-. In addi- 
tion, there is a considerable potential difference across 
the surface membranes of neurons, including all 
their branches, the inside being —50 to —80 milli- 
volts relative to the exterior under normal resting 
conditions. 

It may be claimed that only one hypothesis, which 
may be termed the membrane ionic hypothesis, 
attempts to account quantitatively for propagation 
within neurons both of impulses and of the events 
which control the generation of impulses, and also for 
transmission across synapses. The earliest ionic 
hypothesis was proposed by Bernstein (2) in 1902. 
For the modern version of this ionic hypothesis, as 
applied to the responses within a neuron, reference 
may be made to Hodgkin (49, 50), to Hodgkin & 
Huxley (52) and to Huxley (57). Its application to 
synaptic transmission has been specially developed 
for neuromuscular junctions and the synapses on 
mammalian motoneurons (15, 16, 17, 26, 28, 29, 38, 

4i)- 

Essentially it is postulated that the resting mem- 
brane potential of neurons and muscle fibers ( — 50 to 
— 100 mv) is due to the relatively free diffusion of the 
small ions, K+ and Cl~, across the membrane, while 
the Na+ permeability is of a much lower order. For 
example, in the giant axons of squid the resting K+ 
and Na+ conductances are, respectively, about 0.5 and 
of the order of 0.0 1 mmho per cm-. As a consequence, 
an electrical potential difference is set up across the 
membrane so that there is little or no electrochemical 
potential gradient of the freely diffusing ions, K+ and 
Cl~, across the membrane despite the very large 
concentration differences that obtain, (Ki)/(Ko) and 
(Clo)/CCli), both being of the order of 20 to 50. It 
may be noted that subsidiary hypotheses, such as the 
ionic pump mentioned in the preceding section, are 
required in order to explain how these concentration 
differences are maintained along with the very low 
internal sodium concentration. It is further postulated 
that, if the resting potential of the membrane is sud- 
denly reduced by a considerable amount (say from — 50 
mv to o), both the Na+ and K+ conductances undergo 
characteristic increases. As summarized by Huxley 
(57), the conductance " for Na ions rises in one or two 



tenths of a millisecond to perhaps 15 mmho, cm-, and 
then falls to a low value with a time constant of about 
I msec. That for K ions does not change noticeably at 
first, but rises along an S-shaped curve, becoming 
appreciable as the Na conductance falls from its peak, 
and eventually flattening out and remaining at about 
20 mmho/cm- as long as the membrane potential 
difference is held at zero. When the membrane poten- 
tial difference is restored to its ordinary resting value, 
the K conductance returns to its resting value along an 
exponential decay curve, without an S-shaped start. 
The Na conductance remains low, but the ' inactiva- 
tion' which caused it to fall after its peak during the 
period at zero membrane potential difference per- 
sists, decaying exponentially with about the same 
time constant as the K conductance." Meanwhile the 
Na and K ions have been moving down their electro- 
chemical gradients. For a giant axon there is a gain 
in Na of 3 to 4 X io~'- moles per cm- per impulse and 
a loss of an equivalent amount of K. 

According to the ionic hypothesis, the membrane 
may be represented by an electrical diagram (fig. 2) 
in which the membrane capacitance (a) is shown in 
parallel with two battery-resistance elements (6 and 
(-) representing, respectively, the K and Na difi'usion 
channels across the membrane. The respective 
batteries are at the approximate equilibrium poten- 
tials for K and Na ions, and the resistances which 
represent reciprocals of the respective conductances 
are both capable of variation over a wide range. For 
the .squid axon the respective resistances of the resting 
membrane are about 2 X lo'fi cm^ and lo^fi cm-, 
while during activity the values are as low as 25^ cm^ 
and loi] cm-. 

On the basis of quantitative studies of the time 
courses of the conductance changes as produced by 
a wide range of membrane potential changes, it has 
been possible (52) to set up differential equations 
which relate three parameters to the membrane po- 
tential changes, viz. the 'turning on' of the Na con- 
ductance, the 'turning on' of the K conductance and 
the 'inactivation' of the Na conductance, and in which 
all the coefficients are experimentally determined. 
These equations give a very satisfactory quantitative 
account of a wide range of performance of the giant 
fibers from which the coefficients were derived. It will 
suffice to show how the propagation of the nerve 
impulse is explained. 

The explanation of the propagation of the nerve 
impulse is based on measurements of the cable 
properties of the nerve fiber in addition to the differ- 
ential equation relating the ionic conductances to the 



NEURON PHYSIOLOGY INTRODUCTION 63 



Vnb 



• External fluid 



'\/\/\/\/\/\/\/\/\/\/\/\/\/\/ Inlenor of fibre 



llSmV 




12 mV 



FIG. 2. Theoretical action potential (F) and membrane conductance changes gNa and ^k obtained 
by solving the equations derived by Hodgkin & Huxley (52) for the giant axon at i8.5°C. Inset 
shows diagram of an element of the excitable membrane of a nerve fiber — a, constant capacity; 
b, channel for K+; c, channel for Na+. [From Hodgkin & Huxley (52); Huxley (57)-] 



membrane potential. At any instant the nerve im- 
pulse will be extended as a potential change along 
the nerve fiber as shown in figure 35. According to 
the ionic hypothesis, there will be a net inward move- 
ment of Na ions during the rising phase of the impulse 
(figs. 2, 3.4) because the Na conductance has been 
greatly increased by the depolarization so that Na 
ions move freely down their electrochemical gradient 
carrying positive charges inwards, thus adding to the 
depolarization and hence to the Na conductance. In 
this self-regenerative manner, when the level of 
depolarization of any element of the nerve membrane 
increases above a critical value, it causes the mem- 
brane potential to be carried almost up to the Na 
equilibrium potential which is about +50 mv, i.e. 
internally positive (fig. 2). The delayed development 
of the other two ionic processes checks this potential 
change and eventually restores the resting membrane 
potential; the Na conductance is inactivated and the 
K conductance increases so that, during the falling 
phase of the impulse, the membrane potential is 
dominated by the flux of K ions moving outwards 
along their electrochemical gradient across the 



membrane (figs. 2, 3.-1}, which eventually is restored 
to its original resting potential close to the potassium 
equilibrium potential. Propagation occurs because 
of the cable properties of the nerve fiber, current 
flowing outwards across the membrane ahead of the 
impulse in the circuits, as shown diagrammatically in 
figure 3C. This current efTects a discharge of the mem- 
brane capacitance so that in the zone ahead of the 
impulse the membrane is depolarized sufficiently to 
initiate the regenerative increase in Na conductance, 
by which time the impulse may be said to have 
arrived at this new zone, which will in turn go through 
the conductance changes outlined above. It will be 
appreciated that propagation will be a continuous 
and uniform process along a stretch of nerve with 
uniform properties. The propagation velocity calcu- 
lated from the differential equation and the measured 
cable properties of a nerve fiber is not only of the 
correct order, but is in very close agreement with that 
actually observed (52). Saltatory propagation along 
the nodal structure of a medullated nerve also can be 
satisfactorily explained by the occurrence of essen- 
tially similar processes at each node. This propagation 



64 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



A ^15? 





Fig. 3. A. Diagram showing postulated movement of sodium 
and potassium ions across the membrane during an impulse 
advancing in the direction of arrow, and the resulting alteration 
of charge on the membrane and its recovery. B. Potential 
distribution of the impulse along a nerve or muscle fiber. 
C. Resulting flow of electric current both in the external 
medium and within the fiber. Note the reversal of membrane 
potential during the spike. Figure 3/J is drawn so that the 
impulse is at approximately the same position as in figure 3.4 
and C. 



is treated very iulK in the following chapter by 
Tasaki. 

After the events depicted in figures 2 and 3, the 
ionic hypothesis would predict that a length of nerve 
fiber would have gained a quantity of Na ions that 
was at least adequate to displace the charge on its 
capacitance so that there is the maximum change in 
the membrane potential, and that there would also 
have been an equivalent loss of K ions in the recharg- 
ing process. The actually observed values have been 
several times larger, which is to be expected because 
the periods of Na entry and K emission overlap so that 
much of the ionic influx cancels out as far as the 
membrane potential is concerned. Thus the immediate 
energy source for the propagation of the impulse 
derives from the concentration batteries for Na and K 
ions, and metabolic energy is only later required in 
order to restore the ionic composition. However, the 
ionic flux per impulse is so small relati\e to the ionic 
composition of the fiber that, even in the alj.sence of a 
restorative process, many thousands of impulses can 
be propagated along large nerve fibers without 



significaniK changing the effectiveness of the con- 
centration batteries. 

The ionic hypothesis can also explain satisfactorily 
a great many other properties of nerve fibers [(cf. 
Hodgkin (49); Hodgkin & Huxley C52)], for example 
the subthreshold and threshold phenomena including 
the all-or-nothing behavior, the refractory period fol- 
lowing the impulse, the effects of anelectrotonus and 
catelectrotonus, including accommodation, the effects 
produced on the nerve impulse and the other re- 
sponses by changing the Na or K concentrations, or 
both, in the external medium and in the axoplasm 
(54). This is such an immensely impressive per- 
formance that the ionic hypothesis of the nerve fiber 
must rank as one of the great conceptual achieve- 
ments in biology. 

It is admitted that as yet the ionic hypothesis, in so 
far as it has been formulated, does not give a com- 
plete description of the behavior of the nerve mem- 
brane. For example the nature of the specific changes 
in Na and K conductance is not explained; the in- 
tensity-time courses of changes are merely measured 
and utilized in the explanations. The effect of external 
calcium ions on these conductances also is not yet 
understood. Again, nothing is known about the manner 
in which metabolic energy is employed to drive 
sodium and potassium ions across the membrane 
against their electrochemical gradient. 

As would be expected, such a comprehensive and 
precisely formulated hypothesis has been subjected to 
much critical attack. However much of this criticism 
has been based on imperfectly controlled experiments. 
For example deviations from the predicted effects of 
variations in the external potassium concentrations 
on the resting membrane potential probably have 
been largely due to secondary changes in the internal 
potassium. In this context great significance attaches 
to the recent experiments of Hodgkin & Horowicz 
(51) on the membrane potential of isolated single 
muscle fibers. Extracellular diffusion time is thus re- 
duced to a minimum, so that a steady membrane 
potential is observed within a second of changing the 
external ionic composition and thu« before there is any 
appreciable change in the internal composition. 
Under such conditions, with changes in (Kq), the ob- 
served membrane potentials agree very closely with 
those predicted by the ionic hypothesis. It was also 
remarkable that, making use of the anomalous 
rectification in K ionic diffusion across the membrane 
[cf. Katz (59)], it was possible by changing the internal 
composition of the muscle fiber to have a membrane 



NEURON PHYSIOLOGY INTRODUCTION 



65 



the potential of which was virtually controlled by the 
(C'i„)/(Cli) ratio and then later to restore the normal 
ionic composition of the fiber, as revealed by a normal 
behavior of the membrane potential to variations in 
(K„). 

In conclusion it may he stated that, though detailed 
modifications and developments of the ionic hypoth- 
esis are recjuired in order to explain such phenomena 
as the falling phases of the action potentials of medul- 
lated nerve fibers and cardiac muscle fibers and the 
effect thereon of repolarizing currents, in essentials 
the ionic membrane hypothesis has survived the most 
severe tests and remains as the only conceptual frame- 
work for our discussion of the electrical events that 
are so essentially concerned in all activities of the 
neuron. It will therefore be pertinent to consider now 
the mode of operation of synapses in the light of the 
ionic hypothesis. 



TRANSMISSION BETWEEN NEURONS 

The synapse is a device for the transmission of 
information from one neuron to another. Excitatory 
synaptic action is effective only in so far as it leads to 
the discharge of an impulse by the postsynaptic 
neuron, for only under such conditions does this 
neuron in turn exert effective action on other neurons. 
It may be provisionally concluded from the available 
experimental evidence that any neuron, other than a 
primary sensory neuron, requires excitatory synaptic 
action in order to generate an impulse. In the absence 
of an afferent input even the most complex assem- 
blages of neurons remain silent, as may be seen in the 
isolated cortical slabs of Burns (10). 

On the other hand, inhibitory synaptic action 
attempts to suppress the discharge of impulses and is 
effective in so far as it diminishes or shortens the dis- 
charge produced by any given synaptic excitation. 
Inhibition can be thought of as exercising a sculptur- 
ing role on what would otherwise be the massive 
incoordinate activity of a convulsing nervous system, 
thus reducing it to the organized responses character- 
istic of normal nervous activity. However, just as 
with the excitatory synapses, inhibitory synap.ses 
require activation by presynaptic impulses. Hence, an 
investigation of the transmis.sion between neurons 
can be reduced to a study of the mode of operation of 
excitatory and inhibitory synapses. It will emerge 
that the ionic hypothesis of the nerve membrane 



provides the basis for our atteinpls to understand both 
these types of synaptic activity. 

Excitatory Synaptic Action 

Excitatory synaptic action on neurons is exhibited 
in its simplest form by the monosynaptic action which 
afferent impulses from the annulospiral endings of 
muscle spindles exert on motoneurons. When recorded 
by an intracellular electrode, the monosynaptic 
action by a single volley generates a depolarizing 
potential, the excitatory postsynaptic potential 
(EPSP), that runs virtually the same time course 
regardless of volley size (fig. 4.4 to C). This observa- 




FiG. 4. A to C. EPSP's obtained in a biceps-semitendinosus 
motoneuron with afferent voIley.s of different size. Inset records 
at the left of the main records show the afferent \olley recorded 
near the entry of the dorsal nerve roots into the spinal cord. 
They are taken with negativity downward and at a constant 
amplification for which no scale is given. Records of EPSP are 
taken at an amplification that decreases in steps from A lo C as 
the response increases. Separate vertical scales are given for 
each record of EPSP. All records are formed by superposition of 
about 40 faint traces. D to G. Intracellularly-recorded po- 
tentials of a gastrocnemius motoneuron (resting membrane 
potential, —70 mv) evoked by monosynaptic activation that 
was progiessively increased from D to G. The lower traces are 
the electrically differentiated records, the double-headed arrows 
indicating the onsets of the IS spikes in E to G. HtoK. Intra- 
cellular records evoked by monosynaptic activation that was 
applied at 12.0 msec, after the onset of a depolarizing pulse 
whose strength is indicated in m^ia. A pulse of 20 m^ua was just 
below threshold for generating a spike. H shows control EPSP 
in the absence of a depolarizing pulse. Lower traces give 
electrically differentiated records. Note that the spikes are 
truncated. [From Coombs el at. (14).] 



66 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



tion indicates that each excitatory presynaptic impulse 
generates in the postsynaptic neuron a potential 
change of this same time course, and that the recorded 
EPSP's of figure 4.4 to C are produced by a simple 
summation of these elemental EPSP's. It thus pro- 
vides an illustration of the classical concept of spatial 
summation (72, 73). 

As shown in figure 4D to G, if the EPSP is increased 
beyond a critical threshold level, it causes the neuron 
to discharge an impulse, the latency being briefer the 
larger the EPSP. In figure 4.E, F, G the increase of the 
EPSP to above threshold was brought about by in- 
creasing the size of the presynaptic volley, but, as 
would be expected, the EPSP can also be made to 
generate an impulse by conditioning procedures that 
change the membrane potential towards the critical 
threshold level. For example in figure 4/ to A' the 
same EPSP as in figure 4// was made effective by the 
operation of a background depolarizing current 
which was commenced 1 2 msec, before and which 
changed the membrane potential by the amount 
shown in each record. The impulse is seen to arise 
(at the arrows) at a total level of depolarization of 
about 18 mv, which is made up in varying proportions 
by the conditioning depolarization and the super- 



imposed EPSP. The threshold level of depolarization 
may be attained also by superimposing an EPSP on 
the depolarization produced by a preceding EPSP 
(temporal summation), as is illustrated by Grundfest 
(Chapter \', fig. i 7). 

All these investigations conform with the hypothesis 
that synaptic excitatory action is effective in generat- 
ing an impulse solely by the depolarization of the 
neuron, i.e. by producing the EPSP (17, 28, 29, 44). 
As far as the generation of an impulse by the EPSP is 
concerned, the same processes obtain as with the 
propagation of an impulse from one part of a neuron 
to another. 

In order to produce the EPSP, the activated syn- 
apses must cause an electric current to be generated 
which depolarizes the postsynaptic membrane. Thus, 
as shown in figure ^B, a current must flow inwards 
immediately under the activated synapses, i.e. across 
the subsynaptic membrane, in order that a return 
current may flow outward across the remainder of the 
postsynaptic membrane, so depolarizing it. When a 
brief current pulse is applied across the membrane, it 
builds up a potential difference that on cessation of 
the current decays considerably faster than the EPSP 
(12). Hence it is postulated that the current producing 



V/scc 




o 



NORMAL MEMBRANE 



3x|6'f 



6 



E SYNAPSES 



as low OS 



5x 10 n 



FIG. 5. A. The continuous line is the mean of several monosynaptic EPSP's, while the broken line 
shows the time course of the subsynaptic current required to generate this potential change. B. 
Diagram showing an activated excitatory knob and the postsynaptic membrane. .As indicated by the 
scales for distance, the synaptic cleft is shown at 10 times the scale for width as against length. The 
current generating the EPSP passes in through the cleft and inward across the activated subsynaptic 
membrane. [From Coombs et al. (12).] C. Formal electrical diagram of the membrane of a motoneu- 
ron with, on the right side, the circuit through the subsynaptic areas of the membrane that are 
activated in producing the monosynaptic EPSP. Maximum activation of these areas would be 
indicated symbolically by closing the switch. 



NEURON PHYSIOLOGY INTRODUCTION 



67 



the EPSP is not suddenly switched off after the summit 
of the EPSP, but that, as shown in the analysis of 
figure 5/I (broken line), a small residual current 
continues to flow and thus delays the repolarization 
during the decline of the EPSP (continuous line). It 
will be appreciated that the EPSP's of figures 4 and 
^A are produced by the operation on the neuron of 
the postsynaptic currents generated by many synaptic 
knobs that have been activated simultaneously by 
the afferent volley. 

By passing an extrinsic current across the neuronal 
membrane it has been possible even to reverse the 
potential across it, its interior then being po.sitive to 
the exterior. When this occurs, the EP.SP is also 
reversed in sign (cf. Grundfest, Chapter V, fig. 35), 
which indicates a reversal of the postsynaptic currents 
shown in figure 5^ and of the ionic flux across the 
subsynaptic membrane (17). The effects on the EPSP 
of diminution and reversal of the membrane potential 
and of changes in the ionic composition of the neuron 
are explicable by the postulate that the activated sub- 
synaptic membrane becomes permeable to all small 
ions, such as Na"*", K"*" and Cl~. The time course of 
this permeability change is given by the broken line 
of figure ^A, and its effect on the membrane potential 
can be derived from the electrical diagrain of figure 
5C. A similar investigation on the endplate potential 
of the neuromuscular junction (24, 26; Fatt, Chapter 
VI) has shown that reversal occurs at a membrane 
potential of about — 1 5 mv, which would be close to 
the liquid-junction potential between the muscle fiber 
and its environment. More accurate investigations on 
the EPSP may likewise reveal that a battery of about 
— 1 5 mv should be inserted in the synaptic component 
of the diagram in figure 5C. 

It can now be taken as established that transmission 
across synapses occurs not by the spread of electrical 
currents, but by the specific chemical substances 
which impulses cause to be liberated from the pre- 
synaptic membranes (29, 38, 43). These substances 
alter the ionic permeability of the subsynaptic mem- 
brane and consequently initiate specific ionic fluxes 
across this membrane. These fluxes in turn are re- 
sponsible for the postsynaptic currents that cause the 
transient depolarizations or hyperpolarizations of the 
postsynaptic membrane which are produced respec- 
tively by excitatory or inhibitory action (16, 1 7). Since 
it gives the time course of the ionic permeability 
change, the broken line of figure 5.-I may be taken to 
give the time cour.se of action on the subsynaptic 
membrane of the brief jet of excitatory transmitter 
substance that a presynaptic impulse causes to be 



emitted from the presynaptic knob. .Acetylcholine is 
the transmitter substance at a few types of central 
synapse, but the excitatory transmitter has not yet 
been identified for the great majority. 

Impulses can also be generated in a nerve cell by 
another method that is of particular value in relation 
to the problem of locating the site at which impulses 
arise in nerve cells. When the a.xon of a nerve cell is 
stimulated, an impulse travels antidromically up to the 
nerve cell and usually invades it, generating an anti- 
dromic spike potential as in figure 6A. When thus 
recorded by a microelectrode in the soma, the anti- 
dromic spike potential has two main components, as 
shown by the step on the rising phase which is greatly 
accentuated in the electrically differentiated record 
lying immediately below the potential record in 
figure 6.4. Evidence from recent intensive investiga- 
tions (i, 7, 13, 39, 40, 46) can all be satisfactorily 
explained by the postulate that the initial small spike 
is generated by the impulse in the initial segment of 
the neuron (axon hillock plus nonmeduUated axon), 
while the later large spike is produced when the 
impulse invades the soma-dendritic membrane (13, 
46). The two spikes may therefore be called the IS 
and SD spikes. 

When the neuronal spike potentials generated by 
synaptic or direct stimulation are recorded at suffi- 
cient speed, they are likewise seen to be compounded 
of IS and SD spikes, particularly in the differentiated 
records (fig. 6B), though the separation is always less 
evident than with the corresponding antidromic 
spike potential. It must therefore be postulated that 
the EPSP produced by the activation of synapses 
covering the soma and dendrites is effective not by 
generating an impulse in these regions, but by the 
electrotonic spread of the depolarization to the initial 
segment, as is illustrated by the lines of current flow 
in figure 6C. By recording the impulse discharged 
along the motor nerve fiber in the ventral root it is 
found that usually this impulse started to propagate 
down the meduUated axon about 0.05 msec, after the 
initiation of the IS spike, i.e. the meduUated axon is 
usually excited secondarily to the initial segment (14). 
The critical level of depolarization for generating an 
impulse thus gives the threshold for the IS mem- 
brane, as marked by the horizontal arrow labelled 
IS in figure 65, and not of the SD membrane. An 
approximate measure of the threshold for the SD 
membrane is given by the membrane potential ob- 
tained at the first sign of inflection produced by the 
incipient SD spike, as is indicated by the differentiated 
records in figure 6.4 and B. This potential is measured 



68 HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 

r' 



100 ^ 

ImV 




FIG. 6. Tracings of intracellularly recorded spike potentials evoked by antidromic (.4) and mono- 
synaptic (B) stimulation of a motoneuron, respectively. [From Coombs et al. (14).] The lower traces 
shosv the electrically differentiated records. Perpendicular lines are drawn from the origins of the 
IS and SD spikes, as indicated in the differentiated records, the respective threshold depolarizations 
being thus determined from the potential records and indicated by horizontal lines labelled respec- 
tively IS and SD. C. Diagram showing the lines of current flow that occur when a synaptically 
induced depolarization of the soma-dendritic membrane electrotonically spreads to the initiaj 
segment. 



at the levels of the horizontal SD arrow.s and is 
approximately the same for the antidromically and 
synaptically evoked spikes, as illustrated in figure 'oA 
and B. Synaptic excitatory action thus generates an 
SD spike not directly by its depolarizing action, but 
only indircctK through the mediation of the IS spike 
which lifts the depolarization of the SD membrane to 
threshold by currents that flow in the reverse direction 
to those drawn in figure 6C'. 

With normal motoneurons the threshold level of 
depolarization has always been, as in figure 6.4 and B, 
much higher for the SD membrane than for the IS 
membrane. There has been a consideraljle range in 
the threshold values for motoneurons that are shown 
by their resting and spike potentials to be in good con- 
dition. The IS threshold has ranged from 6 to 18 mv, 
and the SD threshold from 20 to 37 mv (14). However, 
for any one motoneuron the SD threshold has been 
about two to three times the IS threshold. Several 
other types of neurons in the central nervous system 
also reveal a threshold difference between the IS and 
SD membranes. The functional significance of these 
distinctive threshold areas of neurons will be con- 
sidered after synaptic inhibitory action has been 
considered. 

The difference in threshold between the IS and SD 
membranes must not be confused with the concept 



that membranes excited by chemical transmitter are 
inexcitable electrically (cf. Grundfest, Chapter V). 
This concept would be applicable merely to the sub- 
synaptic areas of the SD ineitibrane and not to the 
whole of that membrane. It should be noted that the 
receptor membrane of the bare nerve ending in the 
Pacinian corpuscle also appears to be inexcitable 
electrically, though acting as a primary focus for 
depolarizing the first node of the meduUated axon 
(27; Gray, Chapter IV). There is some analogy here 
with the SD membrane acting to depolarize the IS 
membrane, so generating an impulse there; but the 
analogy does not hold for subsequent e\ents because 
the impulse in the IS membrane usually invades the 
SD membrane, whereas with the Pacinian corpu.scle 
there is no such antidromic invasion. 

Inhibitory Synaptic Action 

Strictlv, the concept of inhibition is restricted to 
depressions of neuronal excitability which occur 
independently of any conditioning excitatory synaptic 
activity on that neuron, and also independently of any 
depression of the excitatory .synaptic bombardment 
that is employed in testing for the suspected inhibition. 
It mav be noted that conditioning by large afferent 
voUevs causes a fairlv prolonged depression in the size 



NEURON PHYSIOLOGY — INTRODUCTION 



69 



of the primary afferent volley and hence depresses its 
excitatory action (8, 45, 55). This effect has been 
attributed to the dorsal root reflex and the dorsal root 
potential set up by the powerful conditioning volley 
(8) and probably is of little significance with more 
physiological types of afferent input. Apart from this 
effect it has been shown that inhibitory actions on 
motoneurons are explained satisfactorily by the tran- 
sient increase which is produced in their membrane 
potentials and which has been designated the inhibi- 
tory postsynaptic potential, IPSP (6, 16, 18). A com- 
parable synaptic inhibitory action has been observed 
with crustacean stretch receptor cells (60), and has 
also been recorded on the neurons of Clarke's column 
by Curtis, Eccles & Lundberg (19a). 

As shown in figure 75 to H, a single volley in the 
afferent fibers from annulospiral endings in quadriceps 
muscle evokes a hyperpolarizing response, the inhibi- 
tory postsynaptic potential (IPSP) in a motoneuron 
of the antagonist muscle (biceps-semitendinosus). The 
IPSP is observed to be increased in a series of stages 
as the afferent volley is increased in size, but it is not 
altered in time course, showing that a simple spatial 
summation occurs when several inhibitory synapses on 
the same neuron are simultaneously activated. With 
the maximum spatial summation in figure jE the 
membrane potential was increased from —60 to 
-63.5 mv. 



In order to produce the observed hyperpolariza- 
tion, current must be flowing inward across the moto- 
neuronal membrane in general, and there must be a 
corresponding outward current in the region of the 
activated inhibitory synapses (fig. 8A, inset). As with 
the excitatory synaptic action in figure 5^, the time 
course of the current that produces the IPSP may be 
determined if the time constant of the membrane is 
known. The broken line in figure 8.4 plots the time 
course so determined and shows that the high intensity 
phase has virtually the same time course as with 
excitatory synaptic action, though there is much less 




r-r-i~r-rT-n 

msec 



Fig. 7. A to H. Lower records give intracellular responses of 
a biceps-semitendinosus motoneuron to a quadriceps volley of 
progressively increasing size, as is shown by the upper records 
which are recorded from the si.xth lumbar dorsal root by a 
surface electrode (downward deflections indicating negativity). 
All records are formed by tfie superposition of about 40 faint 
traces. 




B I ELEMENT ^ ORDINARY 



i-l 



ELEMENT 



I 70 mV I 90 mV | TO mV 

I T . T 



INSIDE CELL 



FIG. 8. A. Continuous line plots the mean time course of the IPSP set up in a biceps-semitendinosus 
motoneuron by a single quadriceps la volley. The measured time constant for the membrane was 
2.8 msec. The broken line gives the time course of the inhibitory subsynaptic current that would 
produce the IP.SP, the calculation being similar to that used in deriving figure ^A. Inset shows lines 
of postsynaptic current flow in relationship to an inhibitory synaptic knob. B. Diagrammatic 
representation of the electrical properties of an ordinary element on the neuronal membrane and of 
an inhibitory element with K+ and Cl~ ion components in parallel. Further description in the 
text. 



70 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



residual action. By investigating the effects of varying 
the membrane potential by current applied through 
the microelectrode (cf Grundfest, Chapter V, fig. 1 2), 
it has been shown that the IPSP is produced by a 
process of ionic diffusion across the subsynaptic 
membrane that has an equilibrium potential at about 
10 mv more hyperpolarized than the resting mem- 
brane potential, i.e. at about —80 mv (16). Further- 
more, it has been shown by ionophoretic injection 
through the microelectrode that this ionic diflusion is 
satisfactorily explained by the hypothesis that the 
inhiijitory synaptic transmitter increases the perme- 
ability of the subsynaptic membrane to ions below a 
critical size, e.g. to K+ and Cl~, and not to somewhat 
larger ions, e.g. to Na+ (16; Grundfest, Chapter V, 
fig. 12). This type of ionic mechanism appears to 
occur with all types of central inhibition so far investi- 
gated and also with the IPSP of the crustacean 
stretch receptor cells (37, 60). It is remarkaljle that 
a somewhat similar ionic mechanism explains the 
vagal inhibitory action on the heart (25, 56, 76) and 
probably for the inhibitory action on crustacean 
muscle (42). 

The electrical diagram in figure 8C illustrates the 
hypothesis that the inhibitory transmitter increases 
the conductance of the subsynaptic membrane to 
both K+ and Cl~ ions, which have the equilibrium 
potentials indicated by the respective batteries, and 
so cau.ses the flow of a current (fig. Bfi) which tends to 
hyperpolarize the rest of the neuronal membrane to 
about —80 mv, which is the mean of the equilibrium 
potentials for K+ and Cl~ ions. 

Factors Controlling Impulse Generation 

The currents which flow from the subsynaptic 
membrane to exert a hyperpolarizing action on the 
motoneuronal membrane and .set up an IPSP (fig. 8 A, 
inset) also effectively hyperpolarize the membrane of 
the initial segment. However the currents generated 
by this ionic mechanism are even more effective in 
checking depolarization (18). On this account, with 
any of the three methods of stimulation, synaptic, 
direct or antidromic, there is an increased difficulty in 
generating an impulse in the motoneuron. All the 
various types of inhibitory action can be sufficiently 
explained by the increased ionic conductance pro- 
duced by the inhibitory transmitter substance and the 
consequent flow of postsynaptic currents that oppo.se 
the excitatory currents [fig. 8; cf Coombs et al. (18); 
Eccles (29)]. 

The low threshold of the initial segment relative to 



the soma-dendritic membrane accounts for the ob- 
servation that with normal motoneurons impulses are 
always generated in the initial segment. As a conse- 
quence the motoneuron acts as a far better integrator 
of the whole synaptic e.xcitatory and inhibitory bom- 
bardment than would be the case if impulses were 
generated anywhere over the whole soma-dendritic 
membrane. If these latter conditions obtained, a 
special strategic grouping of excitatory synapses [cf 
Lorente de No (65)] could initiate an impulse despite 
a relative paucity of the total excitatory synaptic 
bombardment and a considerable inhibitory bom- 
bardment of areas remote from this focus. As it is, 
both e.xcitatory and inhibitory synaptic action are 
effective onlv in so far as they affect the membrane 
potential of the initial segment. It is here that the 
conflict between excitation and inhibition is joined, 
not generally over the motoneuronal surface, as was 
envisaged by Sherrington in his concept of algebraic 
summation. 

In the account so far given the soma-dendritic 
surface functions merely as a generating area for the 
postsynaptic currents that are eff"ective only in so far 
as they act on the initial segment either in generating 
an impulse or in preventing it. If an impulse so 
generated invades the soma-dendritic membrane, it 
does so after the discharge has occurred along the 
axon (14). It might thus appear that the invasion of 
the soma-dendritic membrane is of no consequence in 
the essential function of the neuron in discharging 
impulses down its axon. However, in contrast to the 
initial segment and the medullated axon of neurons, 
the soma-dendritic membrane of many species of 
neurons develops after an impulse a large and pro- 
longed after-hyperpolarization (15, 68). This after- 
hyperpolarization delays the generation of the next 
impulse by the neuron and thus very eflTectively slows 
the frequency of the rhythmic discharges of neurons 
[cf Eccles (28), pp. 174-8]. This frequency control 
by the soma-dendritic membrane is \ery important in 
limiting the frequency with which motoneurons 
activate muscles. Recently it has jjcen shown that the 
motoneurons supplying the slow postural muscles ha\e 
much more prolonged after-hyperpolarizations than 
those supplying the fast phasic mu.scles (30). 

Central Inhibitory Patliivays 

It may be taken as established that at least some 
afferent fibers, e.g. those from annulospiral endings 
and tendon organs, act as pathways both for excita- 
torv and inhibitory actions on motoneurons, and in 



NEURON PHYSIOLOGY INTRODUCTION 



addition exert excitatory actions directly on other 
neurons in the spinal cord (31, 32, 33, 35, 61, 62). 
Until recently values for the central conduction time 
of the so-called direct inhibitory pathway (annulo- 
spiral afferent fibers to motoneurons of antagonist 
action) were derived by measurements of the shortest 
interval at which an inhibitory volley can precede a 
monosynaptic excitatory volley and yet be eflfective in 
inhibiting the reflex discharge. Since such intervals 
approximated to zero, it was erroneously concluded 
that the latency of direct inhibitory action approxi- 
mated to that of monosynaptic excitatory action, and 
hence that the inhibitory pathway was also monosyn- 
aptic, i.e. that the annulospiral afTerents of muscle 
had inhibitory synaptic endings on motoneurons (5, 
28, 61, 64). However the IPSP generated under such 
conditions has a latent period at least 0.8 msec. 
longer than the monosynaptic excitatory action of a 
comparable pathway (35), which is just the interval 
that would be expected if there were a synaptic relay 
on the inhibitory pathway. It has further been found 
that the annulospiral afTerents establish a synaptic 
relay in the interinediate nucleus which conforms in 
every respect with the properties of the direct inhibi- 
tory pathway (35)- Of particular significance is the 
recent observation that the summed action of im- 
pulses in several annulospiral fibers is required before 
any IPSP is produced by them, which contrasts with 
their monosynaptic excitatory pathway, where the 
individual impulses are independently effective in 
generating EPSP (36). Evidently the spatial summa- 
tion of the inhibitory impulses also requires the 
synaptic relay station that has been found in the 
intermediate nucleus and that is required in explain- 
ing the long central latency of the 'direct' inhibitory 
pathway. The same additional latency and inter- 
neuronal relay are observed for the IPSP generated 
through the contralateral inhibitory pathway which 
Wilson & Lloyd (78) have discovered in the Sj and 
S3 segmental levels (20). Finally, the monosynaptic 
excitatory action of afferent impulses from the quadri- 
ceps and gracilis muscles on soleus and biceps-semi- 
tendinosus motoneurons, respectively, (32) provides a 
sufficient explanation of Sprague's observation (74) 
that some afferent fibers entering by the L5 dorsal 
root establish synaptic connections directly with 
motoneurons of the L7 and Si segments [cf. Eccles 
(29), p. 156]. It may therefore be taken as established 
that a single interneuron is interpolated on the direct 
inhibitory pathway, as shown diagrammatically in 
figure g.-l. Similarly there is a single interneuron on 
the inhibitory pathway from motor axon collaterals 



to motoneurons (34), as is shown diagrammatically 
in figure gfi. By a systematic study of the IPSP's pro- 
duced by afferent impulses in the fibers of Golgi 
tendon organs, it has recently been found that there is 
always at least one interneuron on the inhibitory 
pathway, though sometimes two are interpolated (33)- 

Inhibitory and Excitatory Transmitter Substances 

Strychnine has been found to have a highly specific 
and rapid action in depressing inhibitory synaptic 
action (cf. Grundfest, Chapter V, fig. 12), at least 
with the five types of inhibitory action that have so 
far been investigated in the spinal cord (5, 18, 29). 
Similarly, tetanus toxin very effectively depresses all 
these inhibitory synaptic actions (9). In fact the 
clinical effects of both strychnine and tetanus toxin 
can be sufficiently explained by these actions. Since 
the activation of the inhibitory interneurons is not 
affected when synaptic inhibitory action has been 
virtually abolished by strychnine or tetanus toxin, it 
may be concluded that these agents exert their de- 
pressant action on the inhibitory synapses, as indi- 
cated in figure 9.4 and B. On account of the rapidity 
and effectiveness of its action it seems likely that 
strychnine acts competitively with the inhibitory 
transmitter for the receptor patches of the inhibitory 
subsynaptic membrane. Certainly the highly specific 
actions of tetanus toxin and strychnine indicate that 
inhibitory synaptic action is mediated by a specific 
inhibitory transmitter substance. 

The interneuron on the inhibitory pathways (cf. 
fig. 9.4 and E) can be regarded as being introduced in 
order to change over from a neuron that manufactures 
and liberates an excitatory transmitter substance to 
one that operates through the inhibitory transmitter 
substance. It is, therefore, postulated that any one 
transmitter substance always has the same synaptic 
action, i.e. excitatory or inhibitory, at all synapses on 
nerve cells in the mammalian central nervous system. 
According to this principle, any one class of nerve 
cells in the mammalian central nervous system will 
function exclusively either in an excitatory or in an 
inhibitory capacity at all of its synaptic endings, i.e. 
it is postulated that there are functionally just two 
types of nerve cells, excitatory and inhibitory. The 
interneurons illustrated in figure 9.4 and B are ex- 
amples of 'inhibitory neurons'. On the other hand, 
the dorsal root ganglion cells with their primary 
afferent fibers, proi^ably the neurons of all the long 
tracts both ascending and descending, the moto- 
neurons, and many interneurons belong to the class 



72 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 1 




/f 



■ Blocked b/ dihydfo-^-erythroi'j rt 



ToT "^V 



BLOCKED BY STRYCHNINE, TETANUS TOXIN 



MOTO 
NEURONE 



FIG. g. A. Schematic drawing of tiie anatomical and physiological features of the direct inhibitory 
pathway. It shows the events in the primary afferent hber, in its excitatory synaptic connections with 
an intermediate neuron (I cell) and finally in the inhibitory synaptic connection of this neuron with a 
motoneuron, where the inhibitory subsynaptic current is shown by a broken line and the IPSP by 
a continuous line (cf. fig. 6A). B. Diagram summarizing the postulated sequence of events from an 
impulse in a motor axon to the inhibition of a motoneuron. All events are plotted on the time scale 
shown below and the corresponding histological structures are shown diagrammatically to the left 
(note indicator arrows). The four plotted time courses are from above downwards for the following 
events: the electrical response of impulse in motor-axon collateral; the electrical response evoked in a 
Renshaw cell by the cumulatixe effect of acetylcholine at many synapses, showing impulses super- 
imposed on a background depolarization; the IPSP generated in the motoneuron by the Renshaw 
cell discharge; and the aggregate IPSP evoked in a motoneuron that is bombarded repetitively by 
many Renshaw cells, which become progressively more asynchronous, so smoothing the latter part 
of the ripple. The structural diagram to the left shows converging synapses on the Renshaw cell 
and on the motoneuron. [From Eccles c/ at. C34)-j 




'excitatory neurons'. Conceptually, by this subdivision 
of nerve cells into excitatory and inhibitory types, a 
great simplification is produced in the physiology of 
central synaptic mechanisms, for all branches of any 
one neuron can be regarded as having the same 
synaptic function, i.e. as being uniformly excitatory 
or uniformly inhibitory. Terzuolo & Bullock (75) 
give experimental evidence that this principle of 
neuronal specificity does not hold for the cardiac 
ganglion of Limulus. 

In attempting to understand the operation of any 
neuronal system in the mainmalian central nervous 
system, a useful provisional postulate would be that 
all inhibitory cells are short-axon neurons lying in the 
grey matter, while all transmission pathways including 
the peripheral afferent and efferent pathways are 
formed by the axons of excitatory cells. Such a postu- 
late would be of most direct application in relation to 
such simple problems as the modes of termination of 
the descending tracts, but eventually it may be 
applicable also to more complex situations in the 
brainstem and even in the cerebellar and cerebral 
cortices. In all these situations there is as yet no infor- 



mation on the structural features of the inhibitory 
mechanisms. 

It will be sufficiently evident from the above 
account of nerve cells that interactions between nerve 
cells are attributed to synaptic contacts which operate 
by a specific chemical transmitter mechanism. The 
alternate postulate is that, at least in part, interaction 
between neurons is attributable to the flow of electric 
currents generated by active neurons. There is at 
present no experimental evidence that the nervous 
system ot \ertebrates operates in this way. The flow 
of electric currents between neurons is far too small 
to have any significant effect, even in experiments 
using the unphysiologicai procedure of large syn- 
chronous volleys. In contrast it should be mentioned 
that some synapses in Crustacea do operate by elec- 
trical transmission, there being special permeai:)ility 
and rectification properties of the apposed synaptic 
memijranes (47). Such a mechanism would have been 
detected if it were operative at any of the central 
synapses of vertebrates that have been systematically 
investigated. 



NEURON PHYSIOLOGY INTRODUCTION 



73 



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74 HANDBOOK OF PHYSIOLOGY -" NEUROPHYSIOLOGY I 

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CHAPTER III 



Conduction of the nerve impulse 



ICHIJ I TASAKI 



Laboratory of Neurophysiology, National Institute oj Neurological Diseases and Blindness, 
National Institutes of Health, Bethesda, Maryland 



CHAPTER CONTENTS 

Introduction 

Compound Character of Peripheral Nerve 
General Character of the Nerve Impulse 
Cable Properties of the Invertebrate Axon 
Cable Properties of the Myelinated Nerve Fiber 
Conductance of the Membrane During Activity 
Threshold and Subthreshold Phenomena 

Threshold Membrane Potential 

Strength-Duration Relation 

Subthreshold Response 

Measurement of Excitability by Using Test Shocks 
Abolition of the Action Potential 
Nervous Conduction Along Uniform Axon 

Nervous Conduction in Myelinated Nerve Fiber (Saltatory 
Conduction) 

Effect of Increase of External Resistance 

Safety Factor 

Does the Nerve Impulse Jump from Node to Node? 

Field of Potential Produced by a Nerve Impulse 

Conduction in a Polarized Nerve Fiber 

Pfliiger's Law of Contraction 

Effect of Narcosis upon Nervous Conduction 
After-Potentials and Rhythmical Activity 

After-Potentials 

Rhythmical Activity 
Current Theories of the Resting and Action Potentials 

Resting Potential 

Action Potential 



THE MODERN DEVELOPMENT of the coiicept of thc neivc 
impulse may be said to have started with the measure- 
ment of the velocity of the nerve impulse by von 
Helmholtz (141) in 1850. He measured the time 
interval between delivery of an electric shock to the 
nerve of a nerve-muscle preparation and the start 
of contraction of the muscle by two different methods. 
The first method used was to start a constant current 
throuE;h a ballistic galvanometer at the lime of 



delivery of the shock and to interrupt the current 
automatically by a switch opened by the twitch of 
the muscle. The second method he used was based 
on graphical registration of the muscular contraction 
on a moving surface. He compared the time intervals 
measured by stimulating the nerve near its two ex- 
treme ends. 

Helmholtz's finding and the subsequent con- 
firmation and expansion of his observation by a 
number of investigators established the fact that a 
nerve impulse travels along the nerve at a rate far 
slower than that of light or sound in a similar medium 
but substantially faster than the process of transporta- 
tion of substances by streaming or diffusion in a slender 
tube like a nerve fiber. Later, in 1908, Lucas (80) 
found that the velocity of the nerve impulse doubles 
with a rise in temperature of about 10 degrees. The 
question of whether or not the nerve impulse is 
associated with any chemical reacdons, however, 
was not solved until Tashiro (138), Parker (98), 
Fenn (32) and Gerard (40) established the increase 
in production of carbon dioxide and consumption of 
oxygen related to nervous activity. The demonstra- 
tion of heat production associated with propagation 
of nerve impulses by Downing et al. (24) gave a 
further strong support to the view that chemical 
reactions underlie the process of nervous conduction.' 

An entirely different line of approach to the study 
of the processes underlying nervous conduction 
originates with Hermann (47). He worked on a 
'core-conductor model' of nerve which is the prede- 
cessor of the passive iron model (75). The basic idea 

' Some investigators are of the opinion that all the chemical 
reactions take place late in the recovery phase and not during 
the period in which electrical signs of activity can be observed 
[e.g. Hodgkin & Huxley (59)]. 



75 



76 



HANDBOOK OF PHVSIOLOGV ^ NEUROPHYSIOLOGY I 



developed from the observations on the model is that 
nervous conduction may l^e mediated by a flow of 
electric current between successive portions of the 
nerve, i.e. by local circuits. Through very extensixe 
investigations of bioelectricity by Matteucci (86), 
Du Bois-Reymond (25), Biedermann (12) and others, 
it became known that a transient potential variation 
is generated by a stimulation of a nerve between the 
portion of the nerve or the muscle carrying an impulse 
and the killed or resting portion. The existence of a 
local circuit is therefore a logical consequence of the 
direct observations on the bioelectricity of the ner\e. 

A direct demonstration of the decisive role played 
bv a local circuit in the propaeation of an impulse 
was brought forward, a long time after Hermann's 
prediction, first by Osterhout & Hill (95) who worked 
not on the nerve but on a large plant cell, Xitella. 
They found that propagation of an impulse along this 
elongated cell can be reversibly blocked under certain 
experimental conditions by removing or reconnecting 
a salt bridge which constituted a part of the local 
circuit. Later, similar obsersations were made both 
on isolated invertebrate nerve fibers (52) [cf. also (50)] 
and on single nerve fibers of the toad (i 17). 

The development of the concept of the all-or-none 
relationship between the intensity of stimulus and 
the 'size of the response' followed a long, confusing 
course. In 1871, Bowditch (16) found that the 
magnitude of contraction in an excised heart muscle 
of the frog is independent of the intensity of the shock 
used; a weak shock, if effective at all, causes a con- 
traction which is as large as that caused by a strong 
shock. A similar quantal relationship between the 
twitch and stimulus intensity was demonstrated in 
individual muscle fibers of the frog sartorius muscle 
(loi) and also in a ner\-e-mu.scle preparation of the 
frog containing a small number of nerve fibers (81). 
In these cases the 'size of the response' represents 
the magnitude of muscular contraction observed at 
some distance away from the site of stimulation. 

Attempting to expand the concept of 'size of 
response' to include the response in the nerve itself, 
Lucas (82) and Adrian (i) introduced the idea of 
measuring the nerve impulse by its ability to stimu- 
late the adjacent portion of nerve, or by its capability- 
to travel across a narcotized region of nerve — the 
logic being analogous to measuring the power of a 
man by his ability to cross a desert. Through a num- 
ber of ingenious experiments, Lucas and Adrian 
concluded that the size of the nerve impulse in in- 
dividual nerve fibers was independent of the way 
it was elicited. Kato (69) and his associates and also 



Da\is ('/ al. (23) pointed out that there was an er- 
roneous assumption in this argument as to the 
mechanism of narcotic action. However, the con- 
clusion that a propagated ner\'c impulse obeyed the 
all-or-none law turned out to be perfectly correct. 

Another .series of somewhat controversial argu- 
ments was evoked among investigators when the 
concept of 'local' or 'subthreshold' response was 
introduced in physiology. In 1937 Rushton (105) 
predicted the existence of a local response in nerve 
by the following argument. If propagation of a nerve 
impulse is due to successive stimulation of a resting 
portion of ner\-e by the neighboring active (respond- 
ing) area, a definite minimum area of a nerve has 
to be excited by the stimulating current in order that 
the response at the site of stimulation can generate a 
propagated all-or-none response. In other words, 
he stipulates that there should be a 'response' at 
the site of stimulation that is too small to initiate a 
full sized propagating response. 

.Soon after Rushton's prediction, Hodgkin (51) 
obtained clear-cut records indicating the existence 
of 'subthreshold responses' in the invertebrate nerve 
fiber. However, it was found later that Hodgkin's 
demonstration did not prove the legitimac\ of 
Rushton's argument. Cole & Curtis (19) proved that 
the resistance of the surface membrane of the squid 
nerve fiber decreases at the peak of its response 
to about '200 "' the resistance at rest; a responding 
area of the squid axon behaves like a battery with no 
appreciable internal resistance. Lender ordinary 
experimental conditions, it is practically impossible 
to elicit a full-sized response in an area too small to 
initiate a propagated impulse. Furthermore, these 
subthreshold respon.ses were demonstrated in sc)uid 
axons of which a large area was subjected uniformly 
to a stimulating current. Later we shall discuss 
similar phenomena obser\-ed in the nixclinaied nerve 
fiber (p. 98! 

We have discussed up to this poiiu the coin-se of 
development of some of the basic concepts concerning 
the nature of the nerve impulse. We shall describe 
on the following pages the main experimental facts 
known ai)Oul the nerve filler and its ai)ilit\' to carry 
impulses. Emphasis will be placed on the data ob- 
tained from in\ertebrate and vertebrate single nerve 
fibers. There is good reason to belie\e that, at least 
in this field of physiology, the iiehavior of an as- 
sembh' of many nervous elements can be understood 
if the beha\ior of indi\idual fibers under simple, 
well-defined, experimental conditions is known. 
It is generally extremely difiicult to infer the behavior 



CONDUCTION OF THE NERVE IMPULSE 



77 



of individual fibers from observations on the nerve 
trunk. 



COMPOUND CHARACTER OF PERIPHERAL NERVE 

Soon after the first World War, Forbes & Thacher 
(34) introduced a condenser-coupled vacuum tube 
amplifier into the field of electrophysiology. Aided 
by the continued development of electronic engi- 
neering, Gasser & Erlanger (38) in 1922 took the 
first photograph of a ner\e response recorded with 
an instrument ideal in being inertialess. They 
started using a cathode ray oscillograph to register 
the time course of responses of the nerve. 

The standard technique of recording electric signs 
of activity of a whole nerve trunk is to kill (ordinarily 
by crushing) one end of a nerve and to place one 
of the recording electrodes on this killed end (see 
fig. i^). The other electrode needed to measure the 
potential difference is placed on the intact part of 
the nerve near the killed end. Ordinarily, either lightly 
chlorided siKcr wire (abbreviated as Ag-AgCl) 
or calomel half cells (Hg-HgCl) are used for recording 
for they are nonpolarizable. Stimulating electrodes 
(S in fig. i) can be either the Ag-AgCl Ringer type 
or a pair of plain platinum wires. A precaution has 
to be taken to ' isolate' the stimulus from ground, 
namely, to eliminate metallic connection of the 
stimulating electrodes with ground. The main reason 
for the necessity of stimulus isolation is to prevent 
flow of stimulating (and other) currents between the 
stimulating and ground electrodes. The electrodes 
and the nerve are generally mounted in a moist 
chamber to prevent evaporation of water from the 
surface of the nerve. 

The arrangement of the recording electrodes just 
described is called a 'monophasic lead' and a re- 
sponse of the nerve recorded with this arrangement is 
referred to as a 'monophasic action potential'. The 
traditional picture illustrating the principle of this 
method of recording action potentials is as follows. 
The portion of nerve carrying an impulse is 'elec- 
trically negative' to the portion at rest. When an 
impulse started by a stimulus emerges in the region 
of the recording electrode Ei, the potential differ- 
ence between Ei and E2 undergoes a transient vari- 
ation which makes the potential at E2 more positive 
(or less negative) to that at Ei. Since the impulse does 
not reach the region of E2, a potential variation 
representing the ner\ous activity at Ei is recorded 
monophasically. 



The modern picture illustrating the principle of 
monophasic recording (83, 1 24) is slightly different 
from the classical one. Attention is now focused 
upon the nerve fibers and the intercellular space in 
the nerve trunk. When a nerve fiber carries an im- 
pulse, it generates a transient flow of current in the 
surrounding fluid medium. In the region of Ei and 
E2, this transient current in the intercellular space is 
directed from E; to Ei, raising the potential at E2 
relative to Ei for a short period of time. The currents 
produced simultaneously by many fibers in the 
nerve are superposed in the intercellular space and 
give rise to a large coiTipound action potential. In 
this modern picture, the 'electrical negati\ity' in 






40- 



20- 



O.Zmtce 




100 



129 



150-1 



FIG. I. A. Demonstration of the constant velocity of 
propagation of the a- and /3-waves in the action potential of 
the sciatic nerve of the bullfrog. S, the stimulating electrodes; 
El and E», recording electrodes, the latter at the killed end 
of the nerve. The distance from the site of stimulation to the 
recording electrode Ei is indicated on the \ertical line. The 
starting points of the oscillograph trace show the distances at 
which the records were taken. Abscissa, time. [From Gasser & 
Erlanger (38).] B. A similar observation made on a three- 
fiber preparation of the toad. The diameters of the fibers were 
13, 9 and 5 II. The strength of the stimulating shocks employed 
was twice the threshold for the smallest fiber. [From Tasaki 
(124)-] 



78 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



the classical picture- is clearly defined as an IR drop 
in the intercellular space. 

We shall now proceed to discuss the time course of 
the action potential of a buUfroE; sciatic nerve re- 
corded with this arrangement. When the distance 
from the stimulating; electrode (S) to the recording 
electrodes (Ei) is relati\ely large and the shock is 
strong enough to stimulate most of the fibers in the 
nerve, an action potential with several separate 
peaks is observed (fig. i.-!, bottom). As the distance is 
altered, the time intervals between the peaks are 
found to alter, indicating that separate elevations 
in the potential record represent processes travelling 
along the nerve at different velocities. Gasser, Erlanger, 
Bishop and others (13, 38) interpreted these findings 
as resulting from differences in the conduction velocity 
of the different fibers in the ner\'c trunk. 

In figure \B, a set of records is presented showing 
the validity of the interpretation just mentioned. 
Here, the connective tissue sheath around the sciatic 
nerve is removed near its distal end and all except 
three nerve fibers are cut (for the detail of this opera- 
tion, cf. 113, 124). The two recording electrodes 
are placed in two small pools of Ringer's solution 
separated by a narrow air gap (o. i mm wide) across 
which the exposed nerve fibers are mounted. Under 
these circumstances, the electric currents which the 
nerve fibers produce when the impulses arrive at the 
site of recording inevitably flow through the resistor 
QR in the figure) connected between the electrodes. 
The IR drop thus produced is amplified and recorded 
with an oscillograph. 

It is seen in the records that the number of peaks 
observed is equal to the number of the fibers left 
uncut. Three fibers are now generating three sepa- 
rate potential variations. It is also clear that each 
fiber carries an impulse at a rate which is approxi- 
mately constant for the whole length of the sciatic 
nerve. 

It is simple to demonstrate the statistical rule 
formulated by Erlanger and Ga.sser that the con- 
duction velocity increases with increasing fiber 
diameter. If only one large fiber is left uncut, we 
find a high conduction velocity; a weak electric 
shock is sufficient to excite it. If one small fiber is 

- It is important to distinguish a negative potential from a 
negative electric charge. The potential along a uniform electric 
conductor is inevitably related by Ohm's law to a flow of 
current in the conductor; it has to be expressed as a potential 
difference between the two points on the conductor, for in- 
stance, 'the potential of Ei is 10 mv below (or abose) that of 
E;' but not E; is negative and Ei is positive.' 



40 - m/sec 



30 



20 



10 



o 
o o 



o o 



o 

o 



UJ 

> 



z 
o 

I- 
o 
z> 
o 
z 
o 
o 



o 

o 
000 
o 



o 

8 ° 

o 

o o o o 



o 



O o 



FIBER DIAMETER 
J I I I 



8 



10 



12 



14 



16)1 



FIG. 2. Conduction velocity of individual nerve fibers, V, 
plotted against fiber diameter, D. Single fibers were isolated 
from sciatic-gastrocnemius preparations of the bullfrog. The 
outside diameter of the fiber was measured at the operated 
region near the muscle. Temperature, 24°C. [From Tasaki 
et al. (l3i)-] 



isolated in the region of recording, we find a small 
response which arrives at the site of recording after a 
long delay; a strong shock is needed to stimulate 
such a fiber. 

In figure 2 the conduction velocities of about 50 
different fibers in the bullfrog sciatic nerve are plotted 
against their outside diameter. There is a rough 
proportionality between the fiber diameter and the 
conduction velocity, the correlation coefficient be- 
tween the two being 0.92 in this measurement. The 
relation between the minimum effective intensity or 
threshold of shock and the fiber diameter determined 
by this method can be found elsewhere (124). 

It is well-known that the internodal length (the 
distance between the two neighboring nodes of 
Ranvier) increases with the fiber diameter. For the 
fibers in the bullfrog sciatic nerve, the relation be- 
tween the diameter D and the internodal length L 
was found to be expressed by the formula 

L = 0.146 X io'L», 



CONDUCTION OF THE NERVE IMPULSE 



79 



the correlation coefficient between the two being 
0.62. The relation between the conduction velocity 
V (expressed in m per sec.) and the diameter (in fji) 
presented in figure 2 can be expressed by 

V = 2.50/) 

(at 24°C)- From the two formulae abose, it follows 
immediately that 

L 

- = 0.059 (fnsec). 
I 

The ratio L'V represents the average conduction 
time for one internodal length. The last expression 
indicates that, statistically speaking, the internodal 
conduction time is roughly independent of the fiber 
diameter. 

In the experiments involving electric stimulation of 
whole nerve trunks, it is customary to designate 
groups of nerve fibers of different conduction velocities 
as a, /3, 7, (6), B and C. Group a represents the 
fastest myelinated nerve fibers in the nerve with 
velocities of 20 to 30 m per .sec. in the frog, while B 
fibers are the slowest group (5 m per sec. or less) 
at room temperature. The first three (or four) groups 
are often included in .1. Group C represents non- 
myelinated fibers. This cla.ssification is somewhat 
arbitrary. 

The distribution of the fiber sizes in a nerve trunk 
generally shows several peaks of numerical pre- 
dominance. Reflecting this situation, action potentials 
recorded at some distance away from the site of 
stimulation develop sev-eral peaks. However, be- 
cause of the difference in size and duration of the 
action potentials among diflferent fibers, it requires a 
tedious calculation to predict the configuration of 
the action potential of a whole nerve trunk on the 
basis of its fiber size distribution. A detailed treat- 
ment of this problem is found in a monograph by 
Gasser & Erlanger (38). 



GENERAL CH.ARACTER OF THE NERVE IMPULSE 

In the preceding section we have seen an example 
of simplicity and clarity of the experiments done 
with isolated single nerve fibers. It was Adrian & 
Bronk (5) in 1928 who made the first successful at- 
tempt to reduce operatively the number of active 
fibers in a nerve to record single fiber responses. 
The operation of isolating single nerve fibers of the 
frog and the toad was developed in Kato's laboratorv 
(70). 



Another successful approach to single fiber experi- 
ments was achieved by the use of nerve preparations 
of invertebrates, such as crabs, lobsters, crayfish 
or squid. The operative procedure of obtaining single 
fibers in these invertebrate nerves is simpler than the 
dissection of a single frog nerve fiber, since some of 
the fibers in these lower animals are larger than 100 
/i in diameter. So-called squid giant axons, which 
Young (146) has introduced to electrophysiologists, 
are as large as 400 to 900 n in diameter and are an 
excellent material for investigating the potential 
inside the axoplasm. 

Through the use of single fiber preparations, the 
demonstration of some of the basic properties of the 
propagated nerve impulse has become extremely 
simple and direct. The following properties are 
common to all the nerve fibers examined, vertebrate 
and invertebrate. 

a) All-or-none law. The historical aspect of the 
development of this law has been mentioned in the 
introduction of this chapter. This law may be stated 
as follows: with other factors constant, the size and 
shape of any electrical sign of a propagated nerve 
impulse is independent of the intensity of stimulus 
employed to initiate the impulse. 

It has been mentioned that a definite threshold in- 
tensity is needed to initiate an impulse in a nerve 
fiber. As signs of an impulse, one may take the current 
de\eloped by the fiber, the action current, or the 
potential changes inside the axoplasm, or any other 
electrical response of the fiber. When the stimulus 
intensity is varied, these signs may appear slightly 
earlier or later; but the whole time course remains 
uninfluenced by how far above threshold the stimulus 
intensity is. 

The records presented in figure 3 show the time 
course of the action currents produced by a single 
nerve fiber of a toad in response to electric shocks of 
varying intensities. The shocks were applied to the 
sciatic ner\e trunk and the current associated with 
an impulse traveling along a single nerve fiber in the 
nerve was recorded by the technique described in the 
discussion of the experiment of figure iB. At threshold 
(the lowest trace), the action current of the fiber 
started after a long and variable delay. The time 
course of this action current, however, was identical 
with that of the other responses to stronger shocks. 

It is possible to modify the time course of the electric 
response of a fiber by changing physical or chemical 
environmental conditions, such as temperature or 
composition of the fluid around the fiber. This fact 
should not be regarded as a violation of the all-or- 



8o 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 




Fio. 3. Demonstration of the all-or-none behavior of the 
electric response (binodal action current) of a single myeli- 
nated fiber. The distance between the stimulating electrodes 
and the site of recording was 20 mm. The strengths of the 
stimulating shocks were, from the bottom upward, 100, 105, 
150, 200, 250, 300 and 350 per cent of threshold, respectively. 
Time marker, 5000 cycles per sec. Temperature, 20.5°C. 
[From Tasaki (124).] 



none law. This law refers to the identity of the re- 
sponses obtained by changing only the stimulus 
intensity and nothing else. 

The all-or-none law is not applicable, at least not 
in as strict a form as described above, to electrical 
responses recorded at the site of stimulation 
(cf. p. 98). 

/)) The refractory period. The time course of the 
response of a nerve fiber is not influenced bv the rate 
at which the stimulating shocks arc repeated as long 
as the rate is less than about 10 per sec. When, how- 
ever, the repetition rate is increased up to about 100 
per .sec. at room temperature, it is found that the 
responses are difl'crent in their size and shape from 
the responses obtained at lower frequencies. During 
a short period of time after an impulse has swept over 
the fiber, the 'condition' of the fiber is different from 
that of a normal resting filler. This period is called 
the refractory period of the nerve fiber. 

It is customary to investigate the properties of a 
nerve fiber in the refractory period by using a series 
of paired stimuli, a brief conditioning shock followed 
by a brief test shock at an adjustable interval. The 
response of the fiber to the first conditioning shock 
has the normal configuration, while the response to 
the second test shock varies with the time interval 
between the paired shocks. The threshold for the 
second shock is known to undergo a pronounced 
change during the early stage in the refractory period. 

The curve representing the time course of the 



gradual change in threshold with increasing shock 
intervals is generally called a "recovery curve'. In the 
first recovery curve published by Adrian & Lucas (6) 
in 1912, the reciprocal of threshold, the 'excitability', 
was plotted against the interval between the two 
shocks. The thick continuous line in figure 4 shows a 
recovery cur\e determined by using the propagated 
impulses of a single nerve fiber as the index. The 
threshold for the test shock alone (measured i sec. or 
more after the conditioning shock) is taken as unity. 
The oljserved data indicate that, as the interval be- 
tween the conditioning and test shocks decreases, the 
threshold for the test shock rises first gradually and 
then more rapidly. There is a sharp break in the curve 
at the moment when the threshold is about J. 5 to 
3 times the normal \alue, namely, when the excita- 
bility is about 30 to 40 per cent of the normal level. 

This break in the recovery cur\e indicates that, in 
the period following initiation of a propagated nerve 
impulse in a nerve fiber, there is a definite period 
during which the fiber is incapable of carrying a 
second impulse. This period was designated b\' pre- 
vious workers as the 'aljsolutely refractory period', but 
more recently the term the 'least (or critical) interval' 
between two effective stimuli (124, 136) is preferred. 
The reason for this recommendation is the fact that, 
when one determines the recovery curve at the site of 
stimulation, a continuous curve without a break is 
obtained. The term 'functional' absolutely refractory 
period has also been recommended to describe this 
period (103). 

The period during which the excitability recovers 
continuously is called the 'relatively refractory period'. 
Following this period there is often a period of 
heightened excitability which is called the supernor- 
mal phase. During the 'supernormal phase', the size of 
the action potential and the conduction velocity are 
practically normal. 

The thin line in figure 4 shows the recovery curve 
for the same fiber determined at low temperature. 
The temperature-dependence of the recoxery curve 
is pronounced, the Qio being about 3.5 (2, 1 19). The 
effect of temperature change is reversible. 

The conduction velocity is known to be subnormal 
during the relatively refractory period. This is shown 
in figure 5, in which the shock response intervals for 
two impulses were plotted against the distance be- 
tween the site of stimulation and the site of recording. 
The two impulses were set up at an interval slightly 
longer than the least interval of the fiber. It is seen 
in the figure that the shock response interval for the 
first impulse increases proportionately with the con- 



CONDUCTION OF THE NERVE IMPULSE 




10 15 20 

SHOCK INTERVAL 




2 3 4 5 6 

SHOCK RESPONSE INTERVAL 



FIG. 4. Recovery curves of a toad nerse hber determined at two different temperatures. [From 
Tasaki (119).] 

FIG. 5. Relation between the conduction distance and the shock response interval for two impulses 
elicited at an interval of 2 msec. A motor nerve fiber of 1 1 /j in diameter inner\.ating the flexor 
digitorum brevis of the toad. Temperature, 23°C. 



duction distance. Evidently, the first impulse travels 
along the filler at a normal constant rate. 

If the second impulse had travelled at the normal 
velocity, the shock response interval for the second 
impulse should be represented by the dotted line in 
the figure which has the same slope as the straight 
line for the first impulse. Actually, it is seen that the 
observed shock response interval increases with in- 
creasing conduction distance more rapidly than that 
for the first impulse. 

It is easy to figure out the space-time pattern of the 
two impulses based on the experimental data present 
in figure 5. Evidently, the tangent (slope) of the curve 
in the figure represents the velocity of the second im- 
pulse at that moment. At the point where the two 
impulses were initiated, the velocity of the second 
impulse is approximately 50 per cent of the velocity 
of the first impulse. Because of this large difference in 
velocity between the two impulses, the second im- 
pulse lags, spatially and temporally, behind the first 
as they travel along the fiber. As separation between 
the two impulses increases, however, the second im- 
pulse gains more speed because of increasing recovery 
from the refractoriness left behind the first impulse. 
Thus, as they travel along a nerve fiber, the interval 
between the two impulses approaches asymptotically 
a constant value which is independent of the initial 
interval at which they started. 

c) Two-way conduction. It is simple to demon- 
strate that a nerve fiber is capable of carrying im- 
pulses in both directions, from its proximal end 
toward the distal and also in the reverse direction. 
An observation illustrated by figure 6 shows this. 
Here a squid giant axon is used. An entirely analo- 



gous observation has been made on the vertebrate 
myelinated ner\'e fiber. 

The axon is placed in a pool of fresh sea water on 
a glass plate. Near each of the two ends of the axon 
a pair of stimulating electrodes is placed. A recording 
electrode, a glass pipette of about i ix at the tip filled 
with isosmotic potassium chloride solution in this 
case, is pushed into the axoplasm of the axon through 
its .surface membrane. The grounded sea water is 
taken as a reference point for measuring the action 
potential. A stimulus applied at one end, A in the 
figure, gives rise to a response of the all-or-none type, 
indicating that the impulse starting at A trav"els 
toward B. When another stimulating shock is applied 
at the other end, B, sometime after the impulse from 
A has swept o\-er the fiber, the impulse arising at B 
can be recorded by the pipette in the middle of the 
axon (see the top record in fig. 6 ). Since the recording 
pipette can be placed anywhere between A and B 
with essentially the same result, this observation proves 
that the axon is capable of carrying impulses in both 
directions. 

When the time intersal between the shocks at A 
and B is reduced below a certain limit (see the record 
in the middle), the second shock becomes ineffective. 
The explanation of this fact is simple. Soon after 
region B of the axon is traversed by the impulse 
arising at A, this region becomes refractorv and does 
not respond to the second shock. 

What happens if two shocks are applied simulta- 
neously at the two ends A and B? There is no refrac- 
toriness at the site of stimulation in this case since 
these regions have not been traversed by any impulse. 
Hence, an impulse should be initiated at A propa- 



82 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



gating toward B. Simultaneously, another impulse 
starting at B should travel toward A. Then, the im- 
pulses are bound to undergo a collision at a point 
about half way between the two sites of stimulation 
of the axon. After such a head-on collision, it should 
be impossible for the two impulses to travel further 
since the region where the impulse from A is heading 
is freshly traversed by the impulse from B and is conse- 
quently incapable of carrying another impulse. The 
same thing can be said of the region on the other side 
of the site of collision. 

In the bottom record of figure 6, the two stimu- 
lating shocks are delivered in such a way that the two 
impulses collide exactly at the site of recording. This 
is accomplished by adjusting the delays of the two 
shocks after the start of the sweep of the oscillograph 
beam in such a manner that the respon.se to shock A 
alone appears at the same spot on the oscillograph 
screen as the response to shock B alone. Delivery of 
two shocks under these conditions elicits, as can be 
seen in the figure, only one response which has 
almost the same configuration as the respon.se to one 
shock. The shock response interval is known to be 
slightly reduced by collision. A further discussion on 
this topic may be found elsewhere (120). 

d) Multiplication of impulses at the branching point of a 
nerve fiber. Histological studies indicate that vertebrate 
motor nerve fibers frequently undergo dichotomy or 
ramification at nodes of Ranvier, one mother fiber 
giving rise to two (or more) daughter fibers [cf. e.g. 
Eccles & Sherrington (26)]. During the course 
of isolating single nerve fibers innervating the toad 
gastrocnemius muscle, such branching motor fibers 
are sometimes encountered. It has been shown 
in such preparations that the muscle tension developed 
by stimulation of the mother fiber (with two daughter 
fibers intact) is nearly twice as great as the tension 
observed after severing one of the daughter fibers. 
Obviously, this indicates that the iinpulse travelling 
down the mother fiber invades the two daughter 
fibers. By this process of successive dichotomy, an 
impulse travelling along a motor nerve fiber multi- 
plies itself before it reaches a large number of muscle 
fibers. 

Sensory nerve fibers generally dichotomize as they 
approach their peripheral endings. They also branch 
off many collaterals in the spinal cord. It is generally 
believed that impulses multiply themselves at the.se 
bifurcating points. In the squid axons, multiplication 
of impulses at bifurcation points has also been ob- 
served. 

e) Interaction between nerve fibers. When a group 



of fibers in a nerve trunk carries nerve impulses, it 
never happens, under ordinary experimental condi- 
tions, that these impulses are transmitted to the other 
surrounding nerve fibers. This can be shown bv the 
following simple observation. 

The gastrocnemius muscle of the toad or frog is 
innervated by a small nerve twig branching off from 
the large tibial nerve which innervates also plantar 
muscles and the skin of the foot. Stimulation of the 
tibial nerve at a point distal to the exit of the muscle 
nerve to the gastrocnemius does not evoke any po- 
tential variation in the muscle nerve nor any contrac- 
tion in the muscle. Such a stimulus sets up a ' volley 
of impulses' in the majority of the fibers in the tibial 
nerve, but these impulses do not spread to the nerve 
fibers entering the muscle. 

It has been found, however, that there is a very 
weak, barely detectable interaction between the 
nerve fibers in a common nerve trunk. Otani (96) 
found that, when the peroneal branch of the sciatic 
nerve carries a volley of impulses, the threshold for 
the fibers from the tibial branch undergoes a transient 
change. This observation was confirmed and ex- 
panded by several investigators, notably by Marrazzi 
& Lorente de No (85). This result is now interpreted 
on a purely electrical basis: when a group of fibers 
carries impulses, the fluid in the intercellular space is 
traversed by action currents developed by these active 
fibers. If a stimulating current pulse is delivered in 
this region of nerve, the effect of the stimulus is 
modified when it is superposed on or antagonized by 
the action currents. The maximum change in thresh- 
old is of the order of 10 per cent. 

If the mechanism of interaction between nerve 
fibers is electrical in nature, it would be expected that 
the interaction should be greatly enhanced by re- 
ducing the shunting effect of the fluid medium around 
the nerve fiber. Katz & Schmitt (73) have shown 
that this is actually the case. 

The diagram at the top of figure 7 illustrates their 
experimental arrangement. Two nerve fibers of the 
crab were immersed in a pool of mineral oil. Fiber I 
was stimulated with electrodes A and B and its re- 
sponse was observed by means of the recording elec- 
trodes D and E in the figure. At about the time of 
arrival of an impulse from B at the site of recording, 
testing current pulses were delivered through elec- 
trodes C and D to determine changes in threshold of 
fiber II at D. The triphasic curve at the bottom of 
figure 7 is the time course of the threshold changes 
observed. Katz & Schmitt explained these results, 
with good reason, as due to the flow of the action cur- 



CONDUCTION OF THE NERVE IMPULSE 



83 




^ I// h'^ ^ 




FIG. 6. Action potentials of a squid giant axon elicited by 
stimulating shocks at the two ends, A and B, of the axon. The 
recording micropipctte was pushed into the axoplasm through 
the axon membrane. Demonstration of two-way conduction 
{top'), refractoriness (jniddli) and collision of impulses {hol- 
torri). Temperature, '2o°C. (Discussion in text.) 



rent developed by fiber I through the surface mem- 
brane of fiber II. They also demonstrated that the 
velocity of an impulse in fiber II is afTected by the 
impulse in fiber I when the amount of the fluid is 
reduced and when the two impulses are not spatially 
far apart. 

Arvanitaki (9) and Tasaki (124) showed that, 
under special experimental conditions, it is possible 
to make an impulse jump from one fiber to another 
by leading; the action current of one fiber through the 
other. 



CABLE PROPERTIES OF THE INVERTEBR.\TE .^XON 

It is easy to introduce a small glass pipette or a set 
of metal wires longitudinally into a squid giant axon. 
By using such internal electrodes, electric properties 
of the giant axon have been extensively investigated 



by a number of physiologists. We shall discuss in this 
section some of the basic observations which serve to 
clarify electric properties of the resting giant axon 
(fig. 8). 

When a glass pipette electrode of about 100 \l in 
diameter is inserted longitudinally into a giant axon, 
it is found that the potential of this electrode (relative 
to the large ground electrode in the surrounding sea 
water) goes down gradually as the pipette electrode 
is advanced along the axis of the axon. The potential 
inside the axon is negative to (i.e. lower than) that 
of the surrounding fluid medium. When the electrode 
is advanced more than about 10 mm from the point 
of insertion on the surface membrane, the potential 
level of the axoplasm is practically independent of 
the position of the tip of the pipette. In other words, 
the space occupied by the axoplasm is practically 
equipotential. The potential difference between the 
surrounding fluid medium and the axoplasm deter- 
mined by this or other .similar methods is called the 
'resting memljrane potential'. 




I2(H 



Exciubility change in fibre [I 



T 



T 



FIG. 7. Top: Electrode arrangement used for demonstration 
of excitability changes in a single nerve fiber of the crab caused 
by the passage of an impulse in the adjacent fiber. A, B, leads 
for stimulation of fiber I; C, D, leads for stimulation of fiber II; 
D, E, recording leads connecting with amplifier and cathode 
ray oscillograph. Bollom: Excitability changes in fiber II during 
the passage of an impulse in fiber I. Abscissae: time in msec. 
Ordinates: threshold intensity of fiber II in percentage of its 
resting threshold. [From Katz & .Schmitt (73).] 



84 



HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



I y/// //////// //T 




FIG. 8. .1. Resting and action potential of the squid giant axon recorded witli an intracellular glass 
pipette electrode. Time marker (0.5 msec.) indicates the potential level observed when the recording 
electrode was in the surrounding sea water. Temperature, 23°C. B. Exponential variation in the 
meiBbrane potential caused by passage of a constant current through the membrane of a squid giant 
axon with a long intracellular silver wire electrode. The thick portions of the wire in the diagram 
on the top show the exposed surface of the electrodes. Time marker, 1000 cycles per sec. Temperature, 
20°C. (The axons in the diagrams are disproportionately thick and short.) 



The fact that the potential level is the same every- 
where in the axoplasm indicates, according to Ohm's 
law, that there is no measurealjle flow of electric cur- 
rent in the axoplasm at rest. It also proves that the 
resting potential represents, as in the frog muscle 
fiber (76) and in other nervous elements, a sharp drop 
of electric potential across the space occupied by the 
thin surface memijrane of the cell. The resting poten- 
tial of an excised squid giant axon is known to be 50 
to 60 mv; it is considerably smaller than that of verte- 
brate skeletal muscle and nerve cells. 

When a pulse of stimulating current is applied to 
a giant axon with an internal recording electrode, 
there occurs a transient rise of 100 to 120 mv in the 
potential of the axoplasm referred to ground (fig. 8.-1). 
The magnitude of the action potential measured by 
this method is practically independent of the position 
of the electrode tip in the axoplasm. If the tip of the 
internal electrode touches or pierces the surface mem- 
brane, both the resting and action potentials are pro- 
foundly diminished or completeh' eliminated. The 
action potential represents, therefore, a transient 
variation of the potential difference across the surface 



membrane of the axon. It is important to distinguish 
this 'memljrane action potential' from those recorded 
with external electrodes. 

When it was discovered that the membrane action 
potential is suisstantially larger than the resting po- 
tential of the membrane (22, 56), some investigators 
who believed the membrane hypothesis of Bernstein 
(10) were greatly surprised. In 1902 Bernstein postu- 
lated, without clear supporting e\-idcnce, that the 
action potential may be a mere diminution or disap- 
pearance of the resting memijrane potential (see 
p. 117). The finding that the inside potential rises 
above the outside potential near the peak of the ac- 
tion potential, therefore, conflicts with this postulate 
of the membrane hypothesis. 

Besides the role in maintaining a potential differ- 
ence, the surface membrane of the resting axon plays 
another important part in electrophysiology of the 
nerve fiber. The resting membrane has a high re- 
sistance to a direct current. This can be shown by 
the use of the arrangement of figure Bfi, in which a set 
of two metal wire electrodes was used instead of a 
glass pipette. 



CONDUCTION OF THE NERVE IMPULSE 



85 



The electrode set shown in the figure consists of one 
wire with a long (about 1 2 mm) exposed surface and 
the other with a short (i mm) exposed surface. The 
long wire is used to send a constant current into the 
axon and the other for recording potential changes 
caused by the current. The short electrode has its 
exposed (uninsulated) surface in the middle of the 
long one. The remaining surface of each electrode is 
insulated with a layer of enamel. A pulse of constant 
current can be generated by connecting a high 
voltage source to the current electrode through a 
high resistance. 

When the sign of the applied current is such that 
the axon membrane is traversed by an inward di- 
rected current, the potential inside the membrane is 
found to be lowered by the current. However, as can 
be seen in the record in the figure, the potential 
change at the onset of the current is gradual — 
mathematically speaking, exponential. The potential 
change varies roughly proportionately with the in- 
tensity of the applied current. When the current is 
reversed, the sign of the potential change is simply 
reversed, provided that the change in the resting 
potential does not exceed about 5 mv. 

This behavior of the axon can be readily under- 
stood if one assumes that the axon membrane con- 
sists of a condenser with a parallel resistance (fig. gA). 
As is well known, the current flowing through a con- 
denser of a capacity, C, is given by C dV/dt, where 
dV/dt is the rate of change in the potential difference, 
V, across the condenser. The current, /, through a 
system of a conden.ser and a parallel resistance is 



given by the expression 



A 



B 






il 



out 






T 



[) i (!) in "5 ~ 

r,flx V(x-4X,t) V(x,t) y(x+ftx,t) 




dV V 

I = C 1- - , 

dt R 



(4-0 



i.e. by the sum of the capacitati\'e current and the 
ohmic current. When the current, the capacity and 
the resistance, /?, are all constant, the time course of 
the potential is given by 



IR (i - e-"«'0, 



(4-0 



Cm-AX rm/4X 



v = o 



where I is the time after the onset of the current. By 
comparing equation (4-2) with the observed result of 
figure 8B, the values of R and C can be determined. 
The capacity, C, of the giant axon membrane deter- 
mined by this method is approximately i /xf/cm- 
and the membrane resistance is between i and 
2.5 kl2-cm-. [cf. Hodgkin et al. (61), p. 440]. The 
time constant of the membrane, RC, is, therefore, 
I to 2.5 msec.-' 

In the argument developed abo\e, the resistances 
of both the axoplasm and the sea water have been 
ignored. Cole & Hodgkin (20) and Schmitt (ro6) 
have shown that the axoplasm is a homogeneous con- 
ductor with a specific resistance of about 40 ohm -cm 
at 2o°C. The specific resistance of the sea water is 
approximately 20 ohm -cm at the same temperature. 
These resistances are too small to have any observable 
effect upon the measurement of figure QB. 

Now the cjuestion arises of how the voltage .source 
representing the resting membrane potential fits in 
the system of a capacity and a parallel resistance of 
figure 9.-I. It is po.ssible to draw a continuous current 
from the resting membrane; therefore, it is legitimate 
lo represent the source of the resting potential by a 
battery. There are obviously two simple wa\s, B and 
C in the figure, of connecting a battery in the circuit 
ot A. Both ways fit with the obser\ed data. There is 
at present no direct experimental procedure that can 
serve to determine which one of them represents the 
axon membrane better. In the sodium theory (cf 
p. 118), the electromotive force of the membrane is 
assumed to be connected in parallel with the con- 
denser as in B. 

As the result of the above discussion, it has become 
clear that a squid axon behaves like the core-conduc- 
tor of Hermann (see p. 75) or like a submarine cable. 
Using elementary calculus, we may proceed slightly 



FIG. 9. Structure of the squid giant axon revealed by the 
use of intracellular electrodes. C, capacity, and R, resistance 
of the membrane. Two possible ways of connecting the source 
of the resting potential in the circuit of R and C arc shown by 
diagrams B and C. (Further detail in te.xt.) 



^ These figures were obtained by eliminating the effect of 
the current flowing near the end of the current electrode by 
the technique described by Marmont (84). The reader is 
reminded in this connection to pay attention also to the di- 
mensions of these figures. 



86 



HANDBOOK OF PHYSIOLOGV 



NEUROPHYSIOLOGY 



further to discuss the spread of electricity alons a 
uniform resting axon. 

In figure qD, the electric properties of an axon im- 
mersed in a large volume of sea water are represented 
by a network of resistances and capacities. Since we 
are interested only in the change of potentials, the 
batteries are omitted in the figure. The resistance of 
the axoplasm of a unit length is represented by r-,; it 
is related to the specific resistance of the axoplasm 
Ri by the expression 



"■i = 






(4-3) 



where D is the diameter of the fiber. 

Symbols fn, and („, denote, respectively, the resist- 
ance and the capacity of the memljrane covering the 
axoplasm of a unit length. They are related to the 
corresponding figures for a unit area, R„. and Cm, by 
the formulae 

':« = -^ . (4-4) 

c„, = ttDC,,,. (4-5) 

Let V(^x, t) denote the potential of the axoplasm, 
referred to the potential of the surrounding fluid 
medium, at position x and time /. Then the ssmi^ols 
f'(.v — A.v, and ['(v + ^x, can be used to denote 
the potentials at position (,v — A.v) and at (.v -|- A.v), 
respectively. The axon is now imaginati\elv divided 
into a series of .segments of a length lA.v. The a.xoplasm 
resistance (to a longitudinal current) of such a seg- 
ment is then rjA.v. Similarly, the membrane capacity 
and the resistance of one segment are given by 
c„Ax and rn,/A,v, respectively. By applying Ohm's 
law, it is found that the longitudinal current in the 
section labelled i is equal to [("(.v, /) — '(•* — ^x, 0]/ 
(riAAr). Similarly, the longitudinal current through 
■section 2 is equal to [F(.v + A.v, /) — r'(.v, 0]/C''iA.v). 
The difference between the current through i and 
that through 2 is equal to the membrane current, 
which has the form given by equation (4-1). This 
us to the equation 

I^Cv + \x. - r(.v, /) F(.v, - F(.v - Sx.O 



r,Ax 



r\Ax 

dVix, Vix, 

= f„,A.v 1 . 

dl r,^/Ax 



By taking the limit A.v to zcio, we obtain the well 
known cable equation: 



r, a.v2 



dl'Cx, 



It is obvious that the spread of currents in other non- 
myelinated nerve fibers and in a uniform muscle fiber 
can be described by the same equation. 

In the steady state the potential is a function of 
position X alone. Equation (4-6) is then reduced to 



dnXx) 

dx^ 



= VixX 



(4-7) 



in which \\x) represents ['(a:, k). The general solu- 
tion of this equation is 



r(.v) = Aft-^l^- -|-Be+''\ 



C4-8) 



where X, the 'space constant', is related to the mem- 
brane resistance and the axoplasm resistance by the 
expression 

Constants A and B in equation (4-8) depend on the 
boundary conditions. 

In a special case where a constant current of in- 
tensity /o is .sent into the axon at .v = o, constant B 
has to be equal to zero; otherwise, \\x') approaches 
infinity as .v increases. At .v = o where the current is 
sent into the axon, dr(A)'d.v is equal to — '2 ''i^» > the 
factor '2 being introduced to meet the situation where 
the current spreads on both sides of the point .v = o. 
From these boundary conditions, it is found that 
A = ^-n^lfi and B = o. The solution of equation 
(4-7) for this special case is, therefore. 



K*) = }i nX-Zoe- 



C4-10) 



at 



-h Vix, 0. 



(4-6) 



The 'effective' resistance }-2^i^ can be expressed by 
virtue of equation (4-9) as 3^2 ^/^^n^i ■ The space 
constant, X, is a measure of the spread of electricity 
along the axon; the greater the value of X, the more 
extensive is the spread. In the squid giant axon, X is 
of the order of 0.6 cm (20). Solutions of the general 
cable equation for several special cases have been 
achieved (30, 63, 130). 



C'SiBLE PROPERTIES OF THE MYELINATED NERVE FIBER 

Large nerve fibers in the vertebrate nerve have a 
thick layer of fatty substance, the myelin sheath, be- 
tween the cylinder of the axoplasm and the outermost 
layer of connective tissue, the neurilemma or the 
sheath of Schwann. The myelin sheath is broken at 
so-called nodes of Ranvier where the surface of the 
axis cvlindcr is covered dircctlv bv the neurilemma. 



CONDUCTION OF THE NERVE IMPULSE 



87 



The width of the nodal membrane uncovered by the 
myehn sheath is roughly 0.5 to i m- The distance be- 
tween the nodes has been discussed on p. 78. 

The first experimental evidence indicating that the 
myelin sheath has a high resistance to a direct current 
was obtained in Tokyo in 1934 [Kubo, Ono & Tasaki 
cited in (70)]. When the threshold of an isolated 
single nerve fiber was determined with a small 
electrode placed near the fiber, it was found that the 
threshold varied regularly with the distance from the 
nodes of the fiber (fig. 10). In these early experiments 
the threshold was determined by taking twitches of 
the muscle innervated by the isolated fiber as an index 
that a nerve impulse had been initiated in the fiber. 
Later, measurements were made by taking electric 
responses of the fiber as an index [e.g. fig. i in Tasaki 
(123)]. All these experiments indicate that the 
threshold is lowest when the small electrode (stimu- 
lating cathode) is placed directly on one of the nodes 
(the other electrode placed in the fluid medium away 
from the fiber), and is highest when the electrode is at 
the point half way between two neighboring nodes. 
These findings have been interpreted as indicating 
that, because of the high (d.c.) resistance of the 
myelin sheath, the stimulating current enters and 
leaves only at the nodes and consequently that the 
nerve fiber is excited only at the nodes. A further 
discussion on this subject may be found elsewhere 
(71, 124). 

It was found later that the myelin sheath is not a 
perfect insulator but that short current pulses can flow 



readily through this sheath (66, 1:24, 125, 136). To 
illustrate this point, we shall mention an observation 
published in Germany during World War II (136). 
The diagram in figure 11. 4 illustrates the experi- 
mental arrangement used. 

A single nerve fiber of the toad is mounted across 
three small pools of Ringer's solution divided by two 
narrow air gaps of o.i to 0.3 mm width. The pool in 
the middle is about i mm wide and contains only the 
myelin covered part of the fiber. All the nodes (Ni, 
N2 and others) are kept in the larger, lateral pools. In 







\ 



\/ 



FIG. 10. Threshold strength of a long stimulating current 
(in amperes) plotted against distance from a node of Ranvier, 
Ni. Motor nerve fiber of the toad immersed in a shallow pool 
of Ringer. Black circles show the results obtained with the 
cathode of the battery connected to the microelectrode, and 
the crosses with current flowing in the opposite direction. 
Temperature, 23°C. [From Tasaki (124).] 



_n_ 



-T 








No N, N2 



1 


WHtt/t 




t 1 msec 



FIG. II. .-i. Membrane current led through i mm long myelin covered portion of toad motor nerve 
fiber. B. Similar to .1 ; there is a node (Ni) in the middle pool. The fibers were stimulated through 
the electrode on the nerve trunk. Note that the action potential at the node is about 0.9 msec, dura- 
tion at 24°C. (The nerve fiber in the diagram is disproportionately thick and short.) 



88 



HANDBOOK OF PH%SIOLOGY 



NEUROPHYSIOLOGY 



each of the pools, a nonpolarizable electrode is im- 
mersed. The electrodes in the lateral pools are directly 
grounded and the one in the middle pool is grounded 
through a resistor of o. i to 0.3 megohms. The cur- 
rents produced by the fiber in response to an electric 
shock applied to the fiber near its cut end are recorded 
by amplifying the IR drop across the resistor. 

If the myelin sheath were a perfect insulator of 
electricity, no flow ol current should be recorded with 
this arrangement. Actually, a relatively strong flow of 
current is observed through the myelin sheath. As can 
be seen in the records of figure 11.^, the membrane 
current led through the myelin sheath has clear 
double peaks of an outward flow, followed by a long 
phase of a weak inward current. 

When a node of Ranvier is introduced into the 
middle pool (fig. iiE), an entirely different result is 
obtained. The flow of current through the membrane 
of the fiber in the middle pools is triphasic, first out- 
ward, then inward and finally outward (weak). 
Comparing the two records in figure 11, it is found 
that a strong flow of inward current takes place only 
at the nodes of Ranvier. Since the total amount of 
current leaving a fiber at any moment has to be equal 
to the sum of the current entering the fiber at the 
same moment, the peaks of the outward current 
through the myelin sheath (record A) should corre- 
spond roughly to the peaks of inward current at the 
neighboring nodes (Ni and N2). The effects of more 
distant nodes are naturally far smaller than those of 
the neighboring nodes. 

That the first peak in record .-1 of figure 1 1 is caused 
by the response at node Ni and the second peak by 
the response at N2 has been shown in the following 
manner. When a few drops of cocaine-Ringer's solu- 
tion are introduced in the lateral pool in which N2 is 
immersed, the height of the second peak is immedi- 
ately reduced. When the same cocaine-Ringer's solu- 
tion is applied to the portion of the nerve fiber in the 
middle pool, no change in the current is observed. 
Finally, v\hen the narcotizing solution is introduced 
gradually into the pool of Ni, the height of the first 
peak is gradually reduced, while the second peak re- 
mains unchanged until it disappears suddenly at the 
moment when the propagation of the impulse is 
blocked. 

Further evidence indicating that electric responses of 
a myelinated nerve fiber are evocable only at the 
nodes of Ranvier has been obtained by narcotizing 
the portions of the fiber located in the lateral pools 
and stimulating the fiber through two of the elec- 
trodes (124, 132). When there is one node in the 



middle pool (as in the diagram of fig. 11 B), a full- 
sized action current can be recorded from a short (i 
mm) nonnarcotized portion of the nerve fiber. But, 
when no node is left in the normal Ringer's solution 
in the middle pool (as in fig. i lA), no action current 
can be elicited from the fiber. 

The size 01 the membrane action potential at the 
node was estimated by Tasaki & Takcuchi (135) by 
measuring the action current and the resistance of 
the single fiber preparation. Huxley & Stampfli (67) 
estimated it by compensating the action current with 
an external voltage source (assuming that the myelin 
sheath is a perfect insulator). Later, a direct method 
of recording the action potential of the nodal mem- 
brane was developed (128). All the.se indirect and 
direct methods give a figure between 95 and 115 mv 
at the peak of activity. Later, we shall discuss the 
difTerence between the shape of the nodal action 
potential and that of the squid action potential. 

If one assumes that the rnyelin sheath behaves like 
a condenser with a parallel resistance as shown by 
the diagram of figure g.^, the flow of current through 
the myelin sheath should be described by equation 
(4-1) in the preceding section. The voltage I' in the 
equation can be either an applied voltage or an 
action potential developed at the nodes. The two 
peaks in the current flowing through the myelin 
sheath (fig. 11. -1^, therefore, are indicative of the 
situation in which the voltage inside the myelin 
sheath rises in two steps, one step at the beginning of 
the action potential at Ni and the other step when 
X> is also activated. Actually, the time interval be- 
tween the two peaks is close to the internodal con- 
duction time discussed previously on p. 79. 

It requires a slight mathematical treatment of the 
data to separate the current led through the myelin 
sheath into its capacitative and ohmic components 
and to determine the absolute values for the capacity, 
(■„,, and the resistance ;„,, of the myelin sheath (125). 
Although this method of measuring the membrane 
capacity and the resistance is not as direct as that 
for the squid axon, the accuracy of the measurement 
is fairly high (the probable error being about 10 per 
cent). The results of recent measurements of these 
membrane constants are listed in the uppermost 
column of table i. The observed values of f,„ and r,,. 
were converted into the values for myelin sheath of a 
unit area (represented by capitalized figures) by using 
equations (4-4) and (4-5) in the preceding section. 

The capacity and the resistance of the nodal mem- 
brane given in table i were determined by measuring 
the current through node (Ni) in the middle pool of 



CONDUCTION OF THE NERVE IMPULSE 



89 



TABLE I . Resistances and Capacities nf the Myelin Sheath, the Squid Axon and the Nodal Membrane 



farad/cm 



farad/cm* 



ohn 



ohm-cm^ 



Myelin sheath (fiber diameter 12 fi) 
Squid giant axon (diameter 500 ii) 
Nodal membrane 



.6 X lo-i 
.6 X 10-" 
I -5 y-t^^ 



5 X 10-9 

ID"* 

(3-7) X lo-s 



2.9 X 10' 
(6-15) X 10' 

41 Mn* 



10* 

(1-2.5) X io» 
8-20 



Data from references (20, 61, 125). 

* Values for one whole node of Ranvier of the toad motor ner\e fiber. 



figure iii5, alter treatint; this node with a sodium- 
free Ringer's solution or with a dilute cocaine- 
Ringer's solution. The details of the principle of the 
method can be found elsewhere (125). Since it is 
difficult to estimate the area of the nodal membrane, 
the figures for a unit area of the nodal memijrane are 
somewhat inaccurate. For comparison, the membrane 
constants of the squid giant axon are also listed in the 
same table. 

It is interesting to note that the capacity of ijoth 
the myeHn sheath and of the nodal membrane is ex- 
tremely insensitive to changes in the temperature and 
the chemical composition of the surrounding fluid 
medium, while their resistance can be strongly modi- 
fied by slight changes in the environinent C'-4> 125). 
There is, however, one siinple way of increasing the 
capacity of the inyelin sheath, that is, by dissolving 
the fatty substance of the myelin sheath by an appli- 
cation of a saponin-Ringer's solution or some other 
detergent solution. During the early stage of a 
saponin treatment of the myelin sheath, the capacity 
increases as the resistance decreases, the product 
c,„r,„ remaining almost unchanged. This fact strongly 
suggests that the capacity of the inyelin sheath is 
dielectric in nature, determined by the thickness of 
the sheath and the dielectric constant of the myelin 
substance. The dielectric constant of the myelin sheath 
is known to be similar to that of many other fatty 
compounds (66, 125). 



CONDUCT.\NCE OF THE MEMBR.ANE DURING .ACTIVITY 

VVe have seen in the preceding section that the 
development of the action potential represents a tran- 
sient variation in the potential difference across the 
surface membrane of the nerve fiber. In 1939, Cole & 
Curtis C19) demonstrated in the .squid giant axon 
that this variation in the meinbrane potential is asso- 
ciated with a pronounced change in the resistance of 
the membrane. Tasaki & Mizuguchi C'SS) showed a 
similar change in the membrane at the node of Ran- 



\ier. We shall discuss the principle of measuring the 
membrane impedance during activity under relatively 
simple experimental conditions. The method to he 
described is slightly different from that employed by 
Cole & Courtis but the principle is the same. 

In the arrangement shown in the upper part of 
figure 12, a long silver wire electrode about 100 ^i in 
diameter is thrust into a squid a.xon immersed in sea 
water This internal electrode and a large electrode 
immersed in sea water surrounding the a.xon are 
connected to one arm of an alternatino current 




FIG. \i. Measurement of the membrane impedance of a 
squid giant axon during activity with an a.c. impedance 
bridge. The bridge was balanced for the impedance of the 
resting membrane. The two records on the left were taken at 
nearly the same stimulus intensity, but the bridge output was 
amplified 10 times the normal (ix) in the lower record. The 
upper trace in the records displays the unfiltered bridge out- 
put; the potentials recorded are slightly reduced and distorted 
by the bridge. The bridge a.c, 20 kc per sec; temperature, 
22 °C. (Further discussion in text.} 



90 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



Wheatstone bridge. The ratio arms, ri and r-2 in the 
figure, consist of ohmic resistors, r2-ri being io:i or 
larger. The remaining arm consists of condensers 
(C and C) and a resistor (/f). When a high fre- 
quency alternating current is applied to the bridge, a 
sinusoidal potential variation is produced across the 
membrane. By proper adjustment of the variable 
resistance and the capacity, however, it is po.ssible to 
reduce the a.c. output of the bridge to zero. 

As has been mentioned above (p. 85) the axon 
membrane can be represented by a condenser and 
parallel resistance. The relationship between the po- 
tential difference (F) across the membrane and the 
current (/) through the membrane is expressed by 
equation (4-1) which can be rewritten as 



Therefore, 



I = C \- GV, 

di 



(6-1) 



where G is the conductance of the membrane, i.e. the 
reciprocal of resistance R in equation (4-1)- We are 
now interested in the relation between a steady 
sinu.soidal current and a sinusoidal voltage that satis- 
fies equation (6-1). We denote the current by 



/ii sin ut 



and the voltage bv 



V = I'll sin (ail + 9), 



(6-2a) 



C6-2b) 



where /o and i'l, are the current amplitude and the 
voltage amplitude, respectively, co is 27r times the 
frequency and 9 the phase difference between the 
current and the voltage. Introducing (6-2a and b) 
into (6-1), we find that 

/o sin u>t = I'd o>C cos (u/ + d} + GVo sin (_uit + S) 
= Vq (G cos 6 — o>C sin $') sin u/ 
+ Vo (C sin e + uC cos 9) cos oil 

The last equation is satisfied when (and only when) 
the coefficient of cos a;/ is zero and simultaneously 
when the coeflicients of sin oj/ on both sides of the 
equation are equal. This leads to the relations 



(6-3a) 



and 

/u = Vd (C cos e — uC sin 9). 
From equation (6-3a) it follows that 

— (jC G 



h = t'o y/o^C -f- G2 



Fo = /„ 



Vw^C' -I- (? ' 



(6-3b) 



y/ufC'^ -h G2 ' 



V"'C2 -I- G^ 



When the impedance bridge in the upper part of 
figure 12 is roughly balanced for a given intensity of 
the bridge a.c, the current / through the axon mem- 
brane is determined by the variable condensers and 
the variable resistance of the bridge, because r-i » 
ri. Under these conditions, the amplitude \\ is pro- 
portional to the impedance, i /-\/G- -|- co^C', of the 
membrane. When G increases during activity, Fn de- 
creases. In the method involving use of the impedance 
bridge, small changes in the membrane impedance are 
detected by balancing the bridge with the membrane 
impedance at rest and recording small unbalances 
after a high amplification. Under such circum- 
stances, not only a change in the amplitude \'n but 
also any change in the phase d brings about a bridge 
unbalance. When the bridge is at balance, the voltage 
between the two electrodes across the axon membrane 
is completely cancelled by the voltage across ri. A 
change in the phase 6 or in the amplitude Fo , makes 
this cancellation imperfect. 

[In order to detect changes in the membrane im- 
pedance during activity, it is necessary to make the 
frequency of the bridge a.c. high enough so that in 
the period to be examined there are a number of full 
cycles of the a.c. The time resolution in the im- 
pedance measurement is affected also by the char- 
acteristic of the filter circuit in the recording system.] 

.After the Wheatstone bridge has been accurately 
balanced for the membrane impedance at rest, a short 
pulse of outward current is passed through the axon 
membrane. If this pulse is well below the threshold, 
the potential trace (the upper trace in the records of 
fig. 12) shows an exponential decay of the membrane 
potential after the end of the pulse; in this case there 
is very little or no bridge unbalance detectable. When 
the pulse intensity approaches the threshold, the fall 
of the membrane potential after termination of the 
pulse becomes slow and erratic (see p. 98); con- 
comitantly there is a sign of a decrease in the mem- 
brane impedance (record .-1) which can be recorded 
distinctly by increasing the amplification of the a.c. 
bridge output (record B). With supra threshold pulse 
intensities, large unbalances of the bridge are ob- 
served (record C), indicating that there is a marked 



reduction in the membrane impedance associated 
with production of an action potential. 

The temporal relation between the action potential 
and the bridge unbalance shown in record C is similar 
to that observed by Cole & Curtis with their external 
impedance electrodes. They explained their data as 
indicating that at the peak of activity there occurs a 
200-fold increase in the membrane conductance. In 
the squid giant axon, the membrane conductance 
stays above the resting level for some time after the 
end of the falling phase of the action potential. 

In the myelinated nerve fiber of the frog, the im- 
pedance measurement is complicated by the fact 
that the change in the membrane impedance takes 
place only at the node (133). An example of simul- 
taneous recording of the action current and of the 
membrane impedance in a single node is shown in 
figure 1 3. A quantitati\'e analysis of this data is com- 
plicated by the fact that the bridge a.c. flows readily 
through the myelin sheath because of its capacity. 
.Some quantitative information in regard to the con- 
ductance at the peak of activity can be obtained by 
passing testing current pulses through the node and 
comparing the change in the membrane potential due 
to the current pulse before and during activity. It has 
been shown by this method that at the peak of ac- 
tivity the membrane conductance increases approxi- 
mately 10 times. In the nodal membrane, there is a 
close parallelism between the time course of the action 
potential and the time course of the loss in the mem- 
brane impedance (129, 133); in this respect the nodal 
membrane is in sharp contrast with the squid axon 
membrane. 

More recently, Hodgkin, Huxley & Katz (57, 58, 




FIG. 13. Simultaneous recording of action potentials and 
changes in the membrane impedance during activity of a 
single node of Ranvier. In the left-hand record, the bridge was 
balanced for the impedance at rest; in the right-hand record, 
the best balance was obtained near the peak of activity. [From 
Tasaki & Freygang (129).] 



CONDUCTION OF THE NERVE IMPULSE 9 1 

OUT, , IN 

o I o— -oJT. 






-!-^==^^^"»-r 



A2 



XT 



^xouyz^v^^v^ V 



i L 



I 1 

o o— 




Ai 



4: 



4 


|- I 






2 


mA/cm* 




/ 




50 


100 ^ 


° 150 


_ ,. n 1 1 


' f-< 1 


1 


-2 
-4 


\ 

\ 



/ 


V 



FIG. 14. L'/i/)fr.- Arrangement used for clamping the membrane 
potential of a squid giant axon along rectangular time courses. 
This circuit is slightly different from that used by Hodgkin 
et al. (61), but the principle is the same. Ai is a low-gain differ- 
ential amplifier; An, a high-gain differential amplifier (1000 
times). The thick portions of the lines in the axon represent 
the exposed surface of the metal wire electrodes. The distance 
between the two partitions (P) was 8 mm. (The diameter of 
the axon and the wire drawn in the diagram is dispropor- 
tionately large.) Resistance r was 2.5 (sometimes 50 or 250) 
ohms. Lower: Relation between the membrane depolarization 
(F) and the membrane current at the peak of the inward 
surge (/). Near V = o, the V-I relationship is roughly linear, 
but its slope is about '250 °f ''^'" °f 'he straight line on the 
right-hand side. Temperature, 2 2°C. The labile portion of 
the V-I relation shown by the broken line represents either all- 
or-none (probably nonsynchronous) responses in some parts 
of the membrane (the patch theory), or a partial increase in 
the conductance uniformly all o\'er the membrane (the sodium 
theory). 



61) measured in a series of beautiful experiments the 
conductance of the squid axon membrane by a very 
direct, theoretically simple method, often referred to 
as the ' method of voltage clamp'. The diagram in the 
upper part of figure 14 illustrates the principle of the 
method. 

A giant axon is placed across three pools of sea 
water separated by two narrow partitions. A pair of 
metal wire electrodes is thrust through the axon; one 
is used for measuring the membrane potential (F) 
and the other for passing currents through the axon 



9^ 



HANDBOOK OF PHYSIOLOGY' 



NEUROPHYSIOLOGY I 



membrane. The uvo lateral pools are directly 
grounded with large silver wire electrodes. The middle 
pool is also grounded but through a resistor (r) of a 
few ohms. When a current is sent through the l^ng 
internal electrode, this resistor (/■} is traversed by a 
current (/) passing through the axon membrane in 
the middle pool; the small potential drop (/r) is 
amplified and is taken as the measure of the membrane 
current. The membrane potential is measured across 
the axon membrane in the middle pool. The circuits 
connected to the axon are constructed in such a 
manner that the membrane potential (f) can be 
maintained at any desired level by an automatic 
adjustment of the membrane current (/). 

The principle of the automatic control of the mem- 
brane current by the feed-back mechanism is as 
follows. In the diagram of figure 14, Ai is a preampli- 
fier which transmits the membrane potential (I) at 
its input to one of the inputs of a differential amplifier 
A-). The other input of A., marked i in the figure, is 
connected to a source of rectangular (or other) voltage 
pulses. The output of amplifier A2 has the .same 
phase as that of input i and opposite to that of 
input 2. 

First let us consider the case in which input i is 
grounded. When membrane potential (T) tends to 



rise by some intrinsic process in the axon, the poten- 
tial of input 2 starts to rise immediately. This po- 
tential is then amplified and, after reversing its 
polarity, transmitted to the long wire electrode in the 
axon. This immediately causes a flow of an inward 
membrane current which lowers the membrane po- 
tential (r). As a consequence, if the gain of Ao is 
sufficiently high, any change in the membrane po- 
tential (r) can be almost completely suppressed by an 
automatic control of the membrane current (/). In 
practice, the over-all gain of this feed-back amplifier 
was 1000 to 3000. 

Next, we consider the case in which the potential 
of input I of amplifier A2 varies along a rectangular 
time course. .Xt the moment when the potential of 
input I starts to rise, thert is a sudden flow of an out- 
ward current through the axon membrane. This flow 
immediately raises the membrane potential (!'). The 
rise in Fis transmitted to input 2, tending to lower the 
output voltage of A-i. In the steady state there is a flow 
of a constant membrane current which is sufficient to 
maintain the membrane potential at the constant 
level. If the gain of Aj is unity, the time course of the 
membrane potential (T) reproduces the potential 
applied to input i fairly accurately. 

The records furnished in figure 15 show the rela- 




FiG. 15. Relationship between the membrane potential (dotted trace) and the membrane current 
(continuous trace) obser\ed with the arrangement of fig. 14. In records A to D, the membrane po- 
tential was 'clamped' along rectangular time courses by automatic adjustment of the membrane 
current. In E and F, rectangular current pulses were applied through the current electrode and the 
variation in the membrane potential was recorded with the other internal electrode; the defection 
sensitivity of the current trace is 20 times as high as in other records. Blanking of the potential trace 
indicates 0.25 msec. Temperature, 22 °C. 



CONDUCTION OF THE NERVE IMPULSE 



93 



tionslup between the membrane putential and the 
membrane current as revealed Ijy the method of 
voltage clamp. When the membrane potential is 
raised suddenly from its resting le\el to a new level 
slightly above the ordinary threshold (i.e. abo\e 12 
to 15 mv) and is maintained at this constant le\el 
(record ^4), it is found that the membrane is trax'crsed 
by a current which flows first outward, then inward 
and finally outward again. The first phase of the out- 
ward current is .so short that it is .seen as a mere break 
in (he upper (current) trace in the record. The second 
phase of an inward current is seen as a downward 
deflection in the record. The third phase of a steady 
flow of an outward current is shown by the current 
trace staying above the zero level in the right-hand 
side of the record. 

The obvious explanation of the time course of the 
membrane current in records A and B is as follows. 
The a.xon membrane has a capacity of the order of 
I ^f per cm- (p. 85). In order to shift the membrane 
potential suddenly by an amount (', a total charge of 
C- r (where C is the capacity of the memijrane in the 
middle pool) has to be supplied by the current 
electrode. This capacitative flow of current takes 
place within the extremely short period of time during 
which the membrane potential is actually rising. The 
second phase is related to the ability of the membrane 
to produce an action potential in response to a sudden 
rise in the axoplasm potential. If the membrane 
potential had not been clamped (as in fig. 15/^), the 
potential inside the axon should start a rapid rise; an 
inward membrane current is needed to counteract this 
potential ri.se during activity and to maintain the 
membrane potential at the constant level. The third 
phase of the membrane current reflects the situation 
in which a relatively strong continuous current is 
needed to maintain the membrane at a steady 
'depolarized' level. 

When the voltage step in the clamping rectangular 
pulse is increased, the intensity of the inward mein- 
brane current is found to decrease. The relation 
between the depolarizing voltage step and the peak of 
the inward surge of current is plotted in the lower 
part of figure 14. When the voltage step is approxi- 
mately equal to the peak value of the meinbrane 
action potential, the peak of the inward surge is 
found to reach zero (fig. 15C). As the voltage step is 
increased further, the peak stays above the zero 
level; i.e. even at the peak of the inward surge of 
current, the membrane current is in the direction 
imposed by the applied voltage. As can be seen in 
the figure, the relation between the voltage step V 



and the current / at the peak of the inward surge is 
represented by a straight line in a wide range of 
voltage. 

The fact thai the \oltage-current relation is linear 
can he taken as indicating that, in thi^ range of 
membrane depolarization, the axon membrane be- 
haves like a ' battery' with a definite electromotive 
force (emf) and a definite internal resistance. The 
voltage at which there is no current flow represents 
the emf of this i)attery and the slope of the voltage- 
current straight line corresponds to the internal 
resistance. The membrane emf at the peak of the 
inward surge of current coincides with the peak of 
the membrane action potential. In the experiments 
of Hodgkin & Huxley (57, p. 465), the membrane 
resistance determined from the slope of the ] -I rela- 
tion is about 30 ohm -cm'-. The figure obtained 
recently by several investigators from the National 
Institutes of Health is 7 to 12 ohm -cm- (at i5to22°C). 
The resistance of the resting membrane, measured 
with small voltage steps (less than 5 mv or negative 
voltages) is 2 to 3 kl2-cm- (61, p. 440). At the peak of 
activity, therefore, the membrane conductance is 
increased by a factor of one to three hundred.^ 

In agreement with the notion that the inward surge 
of current is associated with the ability of the inem- 
brane to develop an action potential, narcosis of the 
axon with ethanol or urethane is known to eliminate 
the inward surge reversibly. A recently popularized 
method of reversible elimination of the action po- 
tential is to reduce the sodium concentration of the 
.surrounding sea water. 

The finding that sodium ions are necessary in the 
process of excitation is not new. More than half a 
century ago, Overton (97) pointed out that the frog 
nerve-mu.scle preparation loses its ability to respond 
to stimuli unless there are .sodium or lithium ions in 
the medium. He also pointed out that chloride ions in 
Ringer can be replaced with bromide, nitrate, ace- 
tate, salicylate, etc. without eliminating the excita- 
bilit\-. Recently Hodgkin & Katz (62) have shown the 
importance of sodium ions in a inore quantitative 
manner [cf also Huxle>- & Stampfli (68) ]. They have 
found that the spike amplitude of the squid giant axon 

* Quite recently similar voltage-clamp experiments were 
carried out on single node preparations of the toad. It was 
observed that the voltage-current relationship obtained was 
similar to that shown in figure 1 4 except that the labile portion 
of the cur%'e indicated by the broken line was limited in a 
narrower voltage range. The membrane conductance deter- 
mined by this method was approximately 10 times as high as 
that of the resting nodal membrane. 



94 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



decreases with the logarithm of the external sodium 
concentration, the proportionality constant being very 
close to 58 mv which is the coefficient of Nernst's 
equation (cf. p. 1 1 7). 

Based on this and other experimental facts, Hodgkin 
& Huxley (59) formulated a hypothesis in which the 
inward surge is interpreted as the consequence of an 
increase in the membrane permeability specific to 
sodium ions. We shall discuss this point later (p. 1 18). 



THRESHOLD AND SUBTHRESHOLD PHENOMENA 

In the early part of this century when physiolo- 
gists had no way of directly observing the potential 
difference across the excitable membrane, a great 
number of articles were published dealing with the 
problem of threshold excitation of the nerve or the 
muscle. At first, physiologists were charmed by the 
elegant physicomathematical scheme of the ionic 
theory of nerve excitation formulated by Nernst (91). 
He derived the relation between the threshold in- 
tensity of current and its duration on the assuinption 
that excitation took place when the concentration of 
some ion reached a certain critical level near the 
semipermeable membrane of the nerve. Nernst 
argued that the passage of an electric current through 
a uniform electrolytic conductor in the nerve cannot 
bring about any electrochemical changes (except for 
raising temperature) that might be responsible for 
initiation of an impulse. His argument is ba.sed upon 
the principles of electrolytic conductors and un- 
doubtedly it is still valid at present. Nevertheless, 
physiologists .soon abandoned Nernst's approach to 
the problem and accepted more formal, physico- 
chemically vague arguments which reached a climax 
with Monnier-Rashevsky-Hill theory ol nerve excita- 
tion (48, 88, 102). 

At present it is possible to pass rectangular pulses of 
current uniformly through the excitable membrane of 
the nerve fiber, and to determine how the membrane 
potential behaves when the stimulus reaches thresh- 
old. The assumptions adopted by previous investi- 
gators can thus be subjected to direct tests. 

Threshold Membrane Potential 

The excitable membrane at the node of Ranvier of 
the vertebrate myelinated nerve fiber is a narrow 
ring-shaped band. Its width (0.5 to i m) is far smaller 
than the diameter of the fiber at the node or than the 
distance between neighboring nodes. It is possible to 



record potential changes across this membrane by the 
use of a positive feed-back amplifier [e.g. McNichoI 
& Wagner (87)]. 

At the top of figure 16 is shown the experimental 
arrangement used to siud\ the behavior of the nodal 
meiTibrane in threshold excitation. The fiber is 
mounted across three pools of saline solution separated 
by two air gaps. The large pool, where node Nn in the 
figure is immersed, is filled with a dilute cocaine- 
Ringer's solution. The pool in the middle, where the 
node under study, N,, is located, is filled with normal 
Ringer's solution. In the small pool, filled with 
cocaine-Ringer's or an isosmotic pota.ssium chloride 
solution, the small portion of the nerve fiber including 
N; is immersed. The electrode in the large pool is 
connected to a source of a .square voltage pulse. The 
middle pool is grounded, and the smallest pool is 
connected to the high impedance input of a positive 
feed-back amplifier. .Since there is practicalK no 
current in the portion of the fiber in the air gap 
between Ni and No, the potential measured by the 
amplifier approximates the potential drop across the 
nodal membrane of Ni. A rectangular voltage pulse 




FIG. 16. Demonstration of the constancy of the threshold 
membrane potential in stimulation of a single node of Ranvier 
(Ni) with rectangular \oltage pulses (S). Nodes N,, and N2 
are inexcitablc. V indicates the input of a positive feed-back 
amplifier for recording the membrane potential. In each record 
the stimulus intensity and duration used are given. [From 
Tasaki (1^6).] 



CONDUCTION OF THE NERVE IMPULSE 



95 



applied between No and Ni sets up through the mem- 
brane of Ni a current, the time course of which is 
distorted by current flow through the myelin sheath. 

The records in figure i6 show the behavior of the 
membrane potential at threshold as observed with 
this arrangement. The duration of the stimulating 
pulse was varied in the range between 0.05 and 6.4 
msec. At every stimulus duration the stimulus inten- 
sity was adjusted to threshold, and without changing 
the intensity, five to seven sweeps of the oscillograph 
beam were superposed on each record. Because of 
spontaneous variation in the property of the nerve 
fiber (14, 99), the node sometimes responded with a 
full-sized action potential and sometimes failed to 
produce an action potential. 

We may define the 'threshold membrane potential' 
as the highest potential level of the membrane which, 
after the end of the applied stimulating pulse, decays 
without producing an action potential (63, 126). The 
level of the threshold membrane potential measured 
from the resting potential level is often called the 
'threshold (or critical) depolarization.' It is seen in 
the records that the threshold depolarization is 
practically independent of the stimulus duration. 
When the duration is short (e.g. 0.05 msec), a very 
large voltage (200 mv) is needed to excite the node; 
the observed fact is that this high a voltage is required 
to raise the membrane potential within a short period 
of time to the threshold level, which is about 15 mv 
above the resting potential. This is exactly what has 
been assumed in most of the classical theories of nerve 
excitation. 

As we have discus.sed in a previous section, the 
surface membranes of the nerve fiber, both the myelin 
sheath and the nodal membrane, have relatively large 
capacities. Consequently, in order to raise the mem- 
brane potential by a constant amount, higher stimu- 
lus intensities are required at shorter stimulus dura- 
tions. 

However, there is in this type of experiment one 
complication that has not been fully understood by 
previous investigators who worked only on nerve 
trunks. It is the gradual rise in the membrane po- 
tential that precedes the rapid rising phase of the 
action potential in stimulation by a long pulse (see 
fig. 16, record for 6.4 msec). In response to a long 
stimulating pulse, an action potential either appears 
within a few msec, (within 10 msec, at the most) 
after the start of the pulse or fails to appear at all. 
When the action potential fails to appear, the be- 
havior of the membrane potential does not diverge 
from what is expected from the physical constants of 



the resting nerve fiber. When the membrane potential 
starts to diverge distinctly from the simple time 
course, provided that the applied pulse has not been 
withdrawn within 5 msec, or so, there is alw-ays (at 
least in a normal node) an action potential. 

Action potentials evoked by long stimulating 
pulses have a more-or-less gradual rising phase 
followed by a phase of rapid potential rise. If the 
applied stimulating pulse is withdrawn before the 
start of the rapid potential rise, the production of 
a full-sized action potential is prevented. Such a 
gradual potential rise followed liy a sudden potential 
fall caused by a withdrawal of the applied pulse is 
seen in the record labelled 46 mv (1.6 msec.) in 
figure 16. 

The nonlinear phenomenon just described is con- 
sidered at present to indicate the following. The pro- 
duction of an action potential is a kind of 'regenerative' 
or 'autocatalytic' process similar to the explosion 
induced by heating of a mass of gunpowder (105). 
The heat applied from outside causes combustion in 
only some of the gunpowder particles; the heal arising 
from these particles in turn induces combustion in 
other neighboring particles. Similarly, when the 
stimulus duration is sufficiently long, the start of a 
■ response' (the start of comljustion in the analogy 
above) tends to raise the membrane potential (tem- 
perature) together with the applied stimulus (applied 
heat). If the external source of current (heat) is 
maintained, this process eventually raises the mem- 
brane potential (temperature) to a critical explosive 
point. If, however, the applied pulse is withdrawn 
before the critical level of the membrane potential is 
reached, the potential returns to its resting level 
along a variable time course. With very short current 
pulses, the membrane potential has to be raised by 
the external source up to the critical level. ^ 

In the excitation of the invertebrate axon with 
rectangular current pulses, results similar to those in 
figui-e 16 have been obtained by several investigators 
[e.g. Hodgkin & Rushton (63)]. To stress the similar- 
ity between the vertebrate myelinated ner\e fiber 
and the squid a.xon, unpublished records obtained 
by Hagiwara and others are presented in figure 1 7. 
The arrangement of the stimulating and recording 
electrodes used is similar to that in figure 14; two 
metal wires about 30 mm in length were inserted 
along the axis of an axon. Pulses of constant current 

' It should be pointed out that some physiologists have 
slightly different viewpoints in regard to the statement made 
in this sentence (104, 107). 



96 



HANDBOOK OF PHVSIOLOGV -^ NEUROPHYSKJLOGV I 



RECTANGULAR CURRENT PULSES 




SLOWLY INCREASING CURRENT PULSES 



5 msec 




FIG. 17. Upper portion: Stimulation of a squid giant axon by rectangular current pulses applied 
through a long intracellular metal electrode. The membrane potential was recorded with another 
intracellular electrode. Stimulus durations used are indicated by the bars in the records. Lower por- 
tion: Stimulation of a squid giant axon by slowly rising current pulses. The time courses of the current 
pulses used are indicated by the broken lines. [From S. Hagiwara et al., unpublished.] 



were applied through one of the internal wire elec- 
trodes and the change in the membrane potential 
was recorded with the other electrode. Under these 
experimental conditions, the axon memiarane is 
traversed by the applied current uniformly over the 
whole area under investigation. The intensity of the 
stimulating pulses was adjusted to the threshold at 
every stimulus duration. 

It is seen in the figure that the threshold membrane 
potential defined as the highest subthreshold level of 
the membrane potential is approximately constant 
(within about 5 per cent), irrespective of the stimulus 
duration. As in the nodal membrane of the toad 
myelinated nerve fiber, the decay of the membrane 
potential in barely subthreshold stimulation is ex- 
tremely variable. In response to long current pulses 
(see record £)), however, a phenomenon we have not 
discussed before is seen. A barely subthreshold, long 
current pul.se sets up an approximately exponential 
change at the beginning; later, in spite of maintained 
flow of the constant current, the memijrane potential 
is found to fall gradually. This is the behavior of the 
membrane associated with the phenomenon classi- 
cally known as ' accomodation' [see Erlanger & Blair 
(27)]. In the nodal membrane, the process of accom- 
modation progresses more slowly than in the squid 
axon and is not apparent in figure 16. 



It has been known for many decades (79) that a 
slowly increasing current fails to excite a nerve fiber 
even when its intensity rises well abo\e the rheobase.^ 
Evidently, this phenomenon is related to the ' accom- 
modative fall in the membrane potential' just men- 
tioned. This point is illustrated by the records in 
the lower part of figure i 7. When the rate of current 
increase is greater than a certain critical \alue, a 
full-sized action potential starts when the membrane 
potential reaches the threshold level. When the 
membrane current rises .slower than the critical 
rate, the potential begins to fall while the current 
intensity is increasing. Once such an accommodative 
fall in the membrane potential has taken place, the 
potential can rise well above the ordinary threshold 
le\el without initiating an action potential. 

Now, let us turn to the corresponding obsersation 
on the toad myelinated nerve fiber. Figure 18 shows 
the beha\ior of the nodal membrane in threshold 
stimulation bv linearlv rising \oltage pulses. The 
experimental arrangement used is the same as that 
used in the experiment of figure 16. Since there is a 
high ohmic resistance in the axis-cylinder between 

" This is the threshold for a long rectangular pulse. For 
pulses longer than 5 msec, the threshold is practically in- 
dependent of duration. 



CONDUCTION OF THE NERVE IMPULSE 



97 




Fio. 1 8. Variations of the membrane potential of a single 
node (V) caused by linearly rising voltage pulses (S). The 
arrangement of fig. i6 was used. In records A' and B' the miss- 
ing portions of the potential trace (V) indicate production of 
action potentials of about lOO mv in amplitude. The trace for 
the stimulating voltage (S) was blanked at too cps. Large 
motor nerve fiber of the toad. Temperatine, 1 1 °C. [From 
Tjisaki (127).] 

nodes No and Ni and since the time constant of the 
membrane is far shorter than the time scale employed 
in these observations, the time course of the current 
through the nodal membrane is similar to that of the 
applied \0lta5e. In records .-1, B, Cand D, an accoino- 
dative fall in the membrane potential is evident. 
Each of the paired records. A- A' or B-B', was taken 
at almost the same stimulus intensity; in one QA or 5) 
the node failed to respond, and in the other (J' or 
B') a large action potential was evoked. The peak 
value of the subthreshold membrane potential in 
these cases is more erratic than in the experiment of 
figure 16; it is roughly independent of the rise time of 
these stimuli. 

In most classical theories of nerve excitation [e.g. 
Hill C48)], the process of accommodation has been 
regarded as a gradual rise in the threshold level of 
the nerve during the period of prolonged d.c. stimula- 
tion. The direct observations mentioned above 
indicate that this is not exactly the case. It is due to a 
secondary change in the property of the membrane 
which decreases the effectiveness of the current to 
raise the membrane potential. Undoubtedly, this is 
related to the phenomenon of delayed rectification 
described first by Cole (18); he found that the axon 
membrane of the squid shows a resistance to an 
outward directed maintained current far smaller than 



that measured with an inward current [see also 
Hodgkin (53)]. Hodgkin & Huxley (59) attributed 
this process mainly to an increased permeability of 
the membrane to potassium ions. In the nodal mem- 
brane, there is some e\idence indicating that there is 
a change in the resting potential when the membrane 
undergoes an accommodative change (127). 

Strength-Duialion Rilalum 

The relation between the threshold intensity of a 
stimulus and its duration is called a strength-duration 
or intensity-time relation. In the squid giant axon 
excited by means of a long internal metal wire elec- 
trode, the significance of this relation is now very 
clear. When a rectangular pulse of current is applied 
to the membrane through the internal electrode, the 
inembrane potential rises exponentially as described 
by equation (4-2). If a stimulus which lasts no longer 
than about 2 msec' (at i4°C) is to initiate an action 
potential, the membrane potential has to reach the 
critical level, l\, at the end of the pulse. This leads to 
the relation 

F, = IRii - e-J-'Rf), 

in which T is the duration of the current pulse, / is 
the current intensity and RC the time constant of the 
membrane. Rearranging the terms, we have 

/ 

T = RC log . 

This is known as Blair's equation for strength- 
duration relation (15). Because of the interaction 
between the stimulating current and the response of 
the membrane mentioned abo\e, this equation is 
only a poor approximation near the rheobase. 

Stimulation of a squid giant a.xon through a glass 
pipette can be treated in a similar fashion by using the 
solution of the cable equation for the corresponding 
conditions. Again the rheobase will be slightly (20 
to 30 per cent) smaller than that expected from the 
space and time constants of the resting axon mem- 
brane. When the duration becomes far shorter than 
the membrane time constant, another complication 
(related to the phenomenon of abolition of an action 
potential to be discussed in the next section) prob- 
ably sets in. When the current pulse is extremely 
short, the uncharged membrane on both sides of the 
site of stimulation is expected to prevent a further rise 
in potential at the site of stimulation and to suppress 
the start of an action potential. These factors have 
not yet been carefully investigated. 

' This figure was kindly supplied by Dr. S. Hagiwara. 



98 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



In the myelinated nerve fiber, the strength- 
duration relation is determined primarily by the 
complicated network formed by the nodal membrane, 
the axis cylinder and the myelin sheath. Because of 
the interaction between the applied current and the 
start of a response, the rheobase is 20 to 30 per cent 
smaller than that expected from the membrane 
properties at rest (see fig. 16). So far, no one has 
derived the equation describing the distribution of 
the membrane potential caused ' by a rectangular 
voltage applied at one point between two neighboring 
nodes. In practice, however, the strength-duration 
relation is expressed by a purely empirical formula: 



.= .(.+^). 



in which .S' is the threshold voltage, /; the rheobase 
voltage and a a constant which has a dimension of 
time and is known as'chronaxie'. It is known that the 
chronaxie for a node varies markedly with the distance 
between the node under study and the stimulating 
partition (65). 

Subthreshold Response 

It has been shown in the explanations of figures 16 
and 1 7 that the membrane potential raised by a 
brief shock of barely subthreshold intensity decays 
along a variable time course which is far slower than 
that expected from the physical properties of the 
resting nerve fiber. This delay in the fall of the mem- 
brane potential is said to be due to a "subthreshold 
response' or a 'local response'. Such delay occurs only 
when the stimulus intensity is greater than 80 to 90 
per cent of the threshold (in single node preparations). 
This phenomenon is more marked in a preparation 
with high threshold and a poor action potential than 
in a fresh normal preparation. The phase of the 
potential rise in these cases is determined bv the 
physical properties of the resting membrane. The 
subthreshold response is considered as a sign of the 
beginning of the regenerative process which has 
subsided without growing into a full-sized response. 
The historical aspect of the concept of the sub- 
threshold response has been discussed in the intro- 
duction of this chapter. 

A subthreshold 'response' is different from an 
ordinary full-sized response in that it does not leave 
behind it a clear refractoriness. In the period during 
which the membrane potential stays above the level 
of the resting potential, the threshold for the second 
shock (necessary to evoke a full-sized response) is 



lower than the threshold at rest. (In the squid axon, a 
subthreshold response is followed by a small ' under- 
shoot', during which the membrane potential is below 
the resting level; the threshold is higher in this period 
than at rest.) Like an ordinary response, a sub- 
threshold response is associated with a reduction in 
the membrane impedance; the reduction is, however, 
far smaller than that associated with a full-sized 
response (see fig. 12). 

The arriplitude of the full-sized action potential 
depends slightly on whether or not it is preceded by a 
marked subthreshold response. It is seen in the rec- 
ords of figure 16 that the action potentials preceded 
by a slow gradual potential rise are consistently 
smaller than those preceded by an abrupt potential 
rise. Because of this variation in the amplitude of the 
response and of the subthreshold responses, the 
response recorded at the site of stimulation is said to 
be only approximately all-or-none. 

In the experiments of figures 16 and 17, the stimu- 
lating current is applied uniformly through the ex- 
citable inembrane. It is not possible, therefore, to 
interpret the subthreshold response as an action po- 
tential localized in a small area subjected to a strong 
stimulating current (see p. 76). This area hypothesis 
of the subthreshold response can be saved if one 
assumes that the surface of the excitable membrane is 
not uniform but that there are spots or patches where 
the sensitivity to electric stimuli is higher than at the 
remaining surface. In the sodium theory (59), a sub- 
threshold response is attributed to a small increase in 
the sodium conductance of the membrane. 

When a nerve fiber is excited by a stimulating cur- 
rent distributed nonuniformly over the membrane, the 
time course of the subthreshold response is compli- 
cated by the spatial factor. Especially when the state 
of the nerve filler has been altered locally by the 
stimulating or recording electrode or when there are 
large stimulation artifacts, pictures very different 
from those in figures 16 and 17 can be obtained. Be- 
cause of these complications, there have been a num- 
ber of confusing reports on this topic. 

Measurement oj Excitability by L "sing Test Shocks 

In classical physiology writers used to .speak of 
measuring the 'excitability' of the nerve by test 
shocks. Insofar as we define the excitability as the re- 
ciprocal of the threshold (p. 80), this procedure of 
measuring the excitability is simple and straight- 
forward. It seems, however, that to old phy.siologists 
the term 'e.\cital3ilit\' or ' irritabilitv' had some 



CONDUCTION OF THE NERVE IMPULSE 



99 



anthropomorphized meaning [e.g. Verworn (140)] 
and the procedure of measuring it was more-or-less 
comparable to determining a man's ability by 
mental tests. Such a concept of excitability has no 
clear physiological meaning. 

Here, we shall discuss the significance of the method 
of using test shocks to explore the state of the nerve 
fiber. This method has been used mainly on verte- 
brate nerve fibers. 

In the arrangement illustrated in the inset in figure 
19, an isolated nerve fiber is mounted across two pools 
of Ringer's solution. The narrow air gap is located 
between nodes Ni and No. Through the electrodes im- 
mersed on the pools, short pulses superposed on long 
rectangular \-oltage pulses are applied to the fiber. 
The intensity of the short pulse, .S', the voltage of the 
long pulse, V, and the time interval, /, from the begin- 
ning of the long pulse to the start of the short pulse 
are three variables in this experiment. The data pre- 
sented in A were obtained by fixing voltage, v, at one 
of four different values ( — 20, —10, 10 and 20 mv) 
and adjusting S to make the composite stimulating 
pulses barely eflfective in eliciting a nerve impulse at 
varying values of /. The data in B were obtained by 
fixing t at 2 msec, and adjusting v and .S* to make the 
pulses barely effective. Thresholds were determined 
by taking the response of the muscle innervated by 
the nerve fiber as an index of initiation of an im- 
pulse; the same result, however, can \)e obtained by 



taking the action potential of the nerve fiber as an 
index. 

In B, the observed point for .S' = o is at j) = 30 to 
31 mv, indicating that the rheobasic voltage of the 
fiber under these experimental conditions was about 
30 mv. The threshold for the brief shock depends on 
the duration of the shock; for durations shorter than 
about 30 /isec, the threshold rises inversely as the du- 
ration. The shock used in the present experiment was 
within this range and its threshold was taken as unity. 

The curves in A show how the threshold for the test 
shock, S, is modified by the subthreshold pulse, v. At 
any fixed value of /, the change in S is roughly pro- 
portional to I', except when v is greater than about 
50 per cent of the rheobase (5). One thing that looks 
strange in this figure at first sight is the change in 
threshold observed at / = o and for negative values of 
/. This is a constant finding in single fiber experiments 
and has also been observed by Erlanger & Blair in 
their experiments with nerve trunks (27). If the test 
shocks measure the state of the nerve fiber at the 
moment when the shocks are delivered, it is obviously 
absurd that the threshold starts to change before the 
beginning of the subthreshold pulse used to modify 
the state of the fiber. 

There are two factors that serve to explain this 
strange fact. One factor is the time required for the 
spread of membrane potential along the myelin 
sheath, and the other factor is the production of a 



<-t^ 



S/Sc 



1.2 






O y 
/I/ 

■^1.0 



*^. 



0.8 






0.& 



}^ 



Mr 



N, N, 



ZA = -20mV 
-10 



\ 



10 



.A A- 



20 



1- 



-J t 

20 msec 



\ 

I 

0.8 
0.4 



B 



s/s„ 

-1.2 - 

\ 



.s 



I 



2 msec 



,}^ 



:n 



o 1» 



A '-I 



ir 



-20 



20 



40 mV 



FIG. 19. Changes in threshold for a brief shock (S) caused by application of a subthreshold rec- 
tangular voltage pulse (v). The sign of the stimulating \oltage pulse is positi\ e when the pulse induces 
an outward current through node N-j in the diagram. S(, represents the threshold for the brief shock 
alone. 



lOO HANDBOOK OF PHYSIOLOGY ^^ NEUROPHYSIOLOGY I 



subthreshold response. We shall first discuss the lime 
factor. 

When a rectangular pulse of voltage is applied 
across the air gap in the arrangement of figure ig, the 
membrane potential at the nearest node (Ni and Nj) 
rises (or falls) along a sigmoid curve. This sigmoid 
time course arises from the situation that both the 
myelin sheath and the nodal membrane have a 
capacity which delays spread of the membrane po- 
tential. The problem of spread of potential along a 
uniform cable is discu.ssed in some detail on p. 86. 
The situation in the myelinated nerve fiber is compli- 
cated by the discontinuities at the nodes, and un- 
fortunately no rigorous mathematical solution of the 
problem is at present available. It is certain, however, 
that both V (the membrane potential at the node) 
and dV/dl are zero at / = o, and 1' rises first gradually, 
then faster and finally approaches the plateau. 

The variation in the membrane potential caused by 
a brief voltage pulse is given by the derivative, dl'/dt, 
times a constant, because a brief pulse can be re- 
garded as a diff"erence between two long rectangular 
pulses of the same intensity but starting in succession 
at a small time interval. From this it follows that the 
maximum of the membrane potential change caused 
at the node by the test shock is reached a certain 
period of time, t,,, after the delivery of the shock [see 
curve 1-/4 on p. 492 of Lorente de No (77)]. This time 
(to) depends on the distance from the stimulating par- 
tition to the node under study (122). Now, in the 
range of voltage, v, where the relationship between S 
and !> is expressed by straight line I in figure igfi, 
action potentials are elicited when the algebraic sum 
of the potentials caused by v and S reaches the 
critical level. Therefore, the origin of time has to be 
shifted to the left by to if the curves are to represent 
the change in the state of the fiber caused by the sub- 
threshold pulse, V. The argument along this line was 
developed first by Erlanger & Blair (14, 38) and 
later by Tasaki (118, 122). 

Next, we discuss the second factor that has to be 
taken into consideration in the analysis of the curves 
in A of figure 19. W'hcn the test shock, S, precedes the 
start of the subthreshold pulse, v, the change in the 
threshold of S is small. It has been mentioned that, 
when a brief shock is close to its threshold, the fall in 
the membrane potential at the node is far slower 
than that expected from the physical constants of the 
resting nerve fiber. If a weak (positive) rectangular 
pulse (j/) follows such a barely subthreshold test pulse, 
it is possible that the membrane potential is raised 
to the critical level, thus initiating a full-sized response. 



This can account for a decrease in threshold in the 
region where / is negative and v is positive. Katz (72) 
developed this argument to explain the results of his 
experiments in which the effect of a brief shock was 
tested by another brief shock. His argument is not 
entirely correct since he ignored the first (time) factor 
mentioned above. Erlanger & Blair as well as 
Tasaki neglected the second factor arising from the 
subthreshold phenomenon; their argument, there- 
fore, has to be partly modified. 

Finally, we shall discuss the significance of the 
break in the v-S relation in the experiment of figure 
195. Some physiologists believe that this break is a 
sign of the development of subthreshold response to a 
subrheobasic rectangular pulse alone [e.g. Nieder- 
gerke (92)]. As we see, however, in the lower right 
part of figure 16, this is not exactly the case. The 
continuous transition from straight line I to II is evi- 
dently due to the interplay of the two stimuli related 
to the development of ' the slowly rising phase of the 
membrane potential' which precedes a full-sized ac- 
tion potential. 



."iBOLlTION OF THE .ACTION POTENTI.AL 

Initiation of an action potential can be regarded 
as a transition of the membrane from its resting state 
into the active state which is characterized by a low 
membrane resistance and a high potential level. The 
reverse process, i.e. a transition from the active state 
of the membrane to the resting state, was first demon- 
strated in the cardiac muscle of the kid (142), then 
in the toad nodal membrane (126) and finally very 
recently in the squid axon membrane. The action po- 
tential of the cardiac muscle is associated with a 
systolic contraction. The fact that this contraction can 
be abolished by a strong (anodal) current pulse in an 
all-or-none manner has been known since the time of 
Biedermann (12, pp. 257-264). 

The regenerative process of initiating an action po- 
tential is set off by a change (rise) in the membrane 
potential up to a certain level. In an analogous man- 
ner, the process of abolition of an action potential is 
set off by a change (fall) in membrane potential 
down to a critical level. This is shown in figure 20. 
These records were obtained from a single node 
preparation of the toad. The arrangement of the 
stimulating and recording electrodes used is similar 
to that for the experiment of figure 16. The first pulse 
of outward membrane current raises the membrane 
potential to the level slightly above the critical po- 



CONDUCTION OF THE NERVE IMPULSE 




FIG. 20. Abolition of the action potential of a single node by pulses of inward membrane current. 
The lower trace in each record indicates the time course of the voltage applied between No and Ni 
in the diagram of fig. 16, top. The amplitude of the recorded action potential was approximately 100 
mv. Time marks in msec. A toad nerve fiber at io°C. [From Tasaki (126).] 



tential necessary to initiate an action potential. The 
second pulse of inward current is applied during; the 
falling phase of the action potential and lowers the 
membrane potential down to various levels. 

When the change in the membrane potential 
caused by the second pulse is slight (records B, B', C), 
the potential rises after the end of the pulse back to 
the level which might have been reached if the sec- 
ond current pulse had not been applied. When the 
membrane potential is lowered by the second pulse 
below a certain critical level (records C, D), the po- 
tential does not rise after the end of the pulse but falls 
further to the potential level of the resting membrane. 
At the critical intensity of the second pulse (record 
D'), the membrane potential in .some instances rises 
to the level of the active membrane and in others ''alls 
to the level of the resting potential. A further increase 
in the intensity of the second pulse lowers the mem- 
brane potential below the resting potential (record E, 
E'); however, after the end of the pulse, the mem- 
brane potential rises and settles usually at the level of 
the resting potential. 

Similar records of abolition of action potentials 
have been taken from a squid giant axon which has 
been treated with intracellularly injected tetraethyl- 
ammonium chloride. This chemical when applied ex- 



ternally is known to prolong the duration of the ac- 
tion potential of the frog nerve and muscle fiber (46, 
78). Prolonged action potentials of the squid or of 
the toad motor nerve fiber show a remarkable resem- 
blance to the action potential of the heart muscle. 
When the action potential is prolonged as it is in these 
cases, the time constant of the membrane is far 
shorter than the duration of the action potential and 
the demonstration of the phenomenon of abolition is 
thereby made easy. 

It is seen that the critical potential le\el for aboli- 
tion gradually rises during activity. Toward the end 
of the prolonged action potential, the critical level for 
abolition is close to the level of the ' shoulder' of the 
action potential at which the membrane potential 
starts to fall rapidly. 

It is an interesting fact that the action potential 
which has been abolished in its very early phase 
leaves behind it no refractory period. This is shown 
by the superposed record in figure 21. Record A in 
the figure is an ordinary unabolished action potential 
of a single node of the toad motor fiber. When this 
action potential is abolished in its later stages by a 
pulse of inward current through the node (record B), 
there is a relati\ely refractory period following this 
prematurely terminated response; a strong current 



I02 



HANDBOOK OF PHYSIOLOGY -^^ NEUROPHYSIOLOGY 




FIG. 21. Recovery of the amplitude of the action potential 
following abolition of a response of a single node. The ar- 
rangement shown by the diagram in the upper part of fig. i6 
was used. .4. Action potential of a single node (,top~) and a 
truncated 60 cycle wave indicating 100 mv level in applied 
stimulating and abolishing pulses (^bollom). B, C and D. Super- 
posed recordings showing recovery after an abolished response. 
Temperature, io°C. [From Tasaki (126).] 



pulse is needed to initiate a second action potential 
and the amplitude of the second response decreases 
continuously with decreasing interval between the 
two responses. Record D shows that, following; the 
action potential abolished at its peak, the node 
exhibits no refractoriness to the following stimulating 
pulse. In record C, the action potential has been 
abolished after the potential has fallen slightly from 
the peak; it is seen that the amplitude of the second 
response is slightly subnormal at the beginning and 
recovers gradually. 

These observations reveal how the process respon- 
sible for the refractoriness progresses during the falling 
phase of the action potential. As was pointed out by 
Adrian (2) in 1921, the end of the action potential 
coincides roughly with the beginning ot the relatively 
refractory period [cf. Tasaki {119, 124)]. When the 
action potential is abolished in the middle of its falling 
phase, the recovery in the amplitude of the second 
response starts in the middle of the normal recovery 
curve (126). It has been suggested therefore that the 
refractoriness is due to some chemical product which 
accumulates during the falling phase of the action 
potential. In the sodium theory (see p. 1 18) a different 
explanation is given to the origin of the refractoriness. 

The rapid falling phase following the shoulder of a 
normal action potential appears to be a transition of 



the membrane from the active state to the resting 
state resulting from the gradually rising critical level 
for abolition reaching the level of the continuously 
falling potential level of the membrane. 



NERVOUS CONDUCTION .iiLONG UNIFORM .AXONS 

We are now ready to discuss nervous conduction as 
a process that involves production of action poten- 
tials in successive portions of the surface membrane of 
the nerve fiber in an orderly fashion. In the squid 
giant axon, the rise in the membrane potential** at the 
peak of the action potential is 100 to 120 mv and the 
critical depolarization necessary to initiate an action 
potential is 12 to 15 mv. Furthermore, the resistance 
of the membrane in the active area is far smaller 
than that of the membrane at rest (see p. 89). 
Therefore, when a portion of an axon membrane is 
thrown into action by a pulse of stimulating current, 
the adjacent portion of the membrane is automatically 
brought to action Isy the restimulating effect of the 
local circuit between the active and resting area of the 
axon. By a repetition of this process of stimulation 
by the local circuit, the activity spreads indefinitely on 
both sides of the site of initial stimulation. 

The local circuit cannot be closed if there is no 
conducting fluid medium outside the nerve filler. 
Therefore, nervous conduction is expected to stop if 
the saline solution outside the fiber is completely re- 
moved. In practice, it is not possible to remove the 
fluid outside the fiber completely, but it is easy to re- 
duce it by immersing a cleaned single nerve fiber in 
mineral oil. Hodgkin (52) has found that, when an 
isolated nerve fiber of the cralo is immersed in mineral 
oil, the velocity of the nerve impulse is markedly re- 
duced. This is a clear-cut demonstration of the im- 
portance of the local circuit in the process of propaga- 
tion of a nerve impulse. 

In figure 22 a set of records from Hodgkin's paper 
is reproduced. The velocity of the crab nerve fiber in 
normal sea water was 4 to 5 m per sec. This was re- 
duced by 20 to 40 per cent when the fiber was trans- 
ferred into a bath of mineral oil. This reduction in the 

* The membrane potential is defined as the energy required 
to transfer a unit charge across the membrane from the ex- 
ternal medium to the axoplasm. If the potential difference 
between the fluid in the intracellular micropipette and the 
axoplasm (which is probably small but indeterminable) is 
ignored, this coincides with the potential of an intracellular 
electrode referred to the medium. Since the membrane potential 
at rest is a negative quantity, a small rise in the membrane 
potential represents a decrease in its absolute magnitude. 



CONDUCTION OF THE NERVE IMPULSE 



103 




FIG. 22. Demonstration ol the dependence of the conduction 
velocity of a crab nerve fiber upon the resistance of the external 
medium. A and C. Action potential recorded with sea water 
covering 95 per cent of the intermediate conduction distance. 
B and D. Fiber completely immersed in oil. Conduction 
distance, 1 3 mm. Time in msec. [From Hodgkin (52).] 



velocity was prompt and completely reversiijle; there 
seems to be little doubt, therefore, that the effect is due 
to the increased electric resistance of the surrounding 
medium. 

The velocity of a nerve impulse is determined by a 
mechanism involving the interplay of many factors. In 
a uniform axon immersed in a large volume of highly 
conducting fluid medium, the mechanism determin- 
ing the conduction velocity is as follows. In the inactive 
area of the axon ahead of the active area, the mem- 
brane is traversed by an outward current (see fig. 23) 
the intensity of which depends on the velocity of the 
impulse. This current is supplied by the active area 
immediately behind the active-inactive boundary. 
The membrane current in the active area is inward 
(see fig. 23), and this inward current tends to delay 
the rate of potential rise in the active region. If the 
membrane is capable of developing an action poten- 
tial rapidly in spite of the existence of a strong inward 
current, the velocity tends to be high. If the capacity 
and the conductance of the resting membrane are 
large, the active area of the membrane has to supply 
a strong current to bring the membrane potential of 
the inactive area up to the critical level, and conse- 



quently the velocity tends to be small. A large longi- 
tudinal resistance (small fiber diameter) is expected to 
have the same effect upon the velocity as an increased 
external resistance. 

Hodgkin & Huxley (59) determined the relation 
between the membrane potential and the membrane 
conductance on the squid axon. By using the cable 
equation and a set of empirical formulae relating the 
membrane potential and the membrane conductance, 
they calculated the velocity and obtained a solution 
of the right order of magnitude. 

We have discussed in a previous section (see p. 83) 
the cable properties of a uniform invertebrate axon. 
In a uniform axon carrying an impulse of a constant 
velocity, there are certain features that deserve further 
discussion. First of all, it should be pointed out that 
there is an inseparable relationship between the 
spatial distribution of the membrane potential and 
the time course of the action potential. A diagram 
representing the time course of an action potential 
can be converted into a diagram showing the spatial 
distribution simply by converting the time scale into 
the distance scale by using the conduction velocity as 
a conversion factor. This and the following statements 
are not applicable to axons with any macroscopic 
nonuniformity along their length in regard to the 
size and shape of the action potential. 

Next to be discussed is the relationship between the 
spatial distribution of the action potential and the dis- 
tribution of the longitudinal and the membrane cur- 
rent of the axon. According to Ohm's law, the longi- 
tudinal current in the axoplasm is proportional to the 
gradient of the potential in the axoplasm, i.e. 



_ -I dV 
/"i dx 

-_i dV 

r,v at ' 



(9-1 a) 



(9-ib) 



where h is the longitudinal current, r, the axoplasm 
resistance per unit length of axon, V the potential of 
the axoplasm (as a function of time, t, and distance 
along the axon, .v) and u the conduction velocity. A 
variation in the longitudinal current with respect to 
space is associated with the membrane current, /„, 
(Kirchoff's law), i.e. 

(9-2 a) 
(9-2b) 

(9-2c) 





a/i 








ax 




I a^v 




ri dx^ 




I a^v 


^ 


)-i02 dt^ 



104 



HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



100 


-V 


r-N Fig 23 


lm..c 


mV 


15 mm 


50 


- 




MEMBRANE 
POTENTIAL 





•////////////\, 

















J_ 



-100 




IMPULSE 



Fig. 24 



r\f 1 

-J V 



V ^ 



■^ 



'' X N- //////iy///////// . 

'''/////,V.\//////^/- 



LONGITUDINAL 
CURRENT 




FIG. 23. Diagrams showing tiie spatial and temporal distribution of the membrane potential, V, 
the longitudinal current, li, and the membrane current, Im- The curves for I^ and Im were obtained 
from the upper curve for V by the graphical method of determining derivatives. 

FIG. i\. Simultaneous recording of the membrane action potential (V) and the membrane cur- 
rent (Im). The width of the middle pool was about 2 mm. The potential drop across the resistor r 
was taken as the measure of the meinbrane current. Temperature, 20°C. 



These equations show that the membrane current of 
a uniform axon is proportional to the second deriva- 
tive (with respect to either time or space) of the mem- 
brane action potential [cf. Katz & Schmitt (73)]. It 
should be pointed out in this connection that equa- 
tions C9-2) were derived without any assumption as to 
the behavior of the resting or active membrane. These 
equations fail to hold only when the axoplasm dis- 
obeys Ohm's law or when the propae;ation of the im- 
pulse is macroscopicalh' nonuniform. 

Figure 23 shows the space and time patterns of the 
membrane potential, the longitudinal current and the 
membrane current as calculated by equations (9-1) 
and (9-2). To emphasize the similarity between the 
space pattern and the time course of the action po- 
tential, the impulse is assumed in this figure to travel 
from the right-hand end of the axon to the left. The 
resistance r; is assumed to be 1.5X10^ ohm per cm 
[cf. Schmitt (106)] and the velocity to be 15 m per sec. 
It is seen that the longitudinal current is diphasic and 
the membrane current is triphasic. It is simple to 
prove that the total area under the curve for the longi- 



tudinal current or under the curve for the memijrane 
current has to be equal to zero. 

The upper part of figure 24 shows an approximate 
method of recording the membrane current of the 
giant axon of the squid. A giant axon is mounted 
across three pools of sea water separated by two nar- 
row partitions. The large lateral pools are directly 
grounded, and the small middle pool is grounded 
through a small resistor. The membrane current flow- 
ing through the portion of the fiber in the middle 
pool is measured by amplifying a small potential drop 
across the resistor between the middle pool and 
ground. In order to obtain a simultaneous recording of 
the membrane action potential, a microelectrode is 
inserted into the portion of the axon in the middle 
pool. The axon is excited by a shock applied near its 
end. The record presented in the figure shows that 
the temporal relation between the action potential 
and the membrane current is very similar to what has 
been expected from the results of the calculations in 
figure 23. 

W^e shall now discuss the field of potential in the 



CONDUCTION OF THE NERVE IMPULSE 



surrounding fluid medium produced by the triphasic 
membrane current just mentioned. If the space-time 
pattern of the membrane current is given, the problem 
of finding the potential field in a volume conductor is 
a purely physical problem, namely, an application of 
Ohm's law to the electrolytic conductor around the 
axon. 

The simplest example of problems of this type is the 
case in which a uniform axon is surrounded through- 
out its length by a conducting fluid of a uniform thick- 
ness (fig. 25/I). We assume that the volume of fluid 
is not so small as to modify the spatial distribution of 
the membrane current di.scussed above. Let s denote 
the resistance per unit length of the surrounding fluid 
medium; in the present case, s « ^i, where r-, is the 
resistance per unit length of the axoplasm. We express 
the potential diflerence across the axon membrane at 
point .V and time / explicitly as l^x + vO, indicating 
that the variation in the membrane potential travels 
leftward at a constant velocity, v. Similarh, the 
longitudinal current and the membrane current are 
functions of (.v -{- vt). 

It is simple to show that the total current flowing 
through the whole cross section of the surrounding 
fluid medium at any point, .v, at any moment, t, is 
equal and opposite to the longitudinal current in the 
axon at the same x and /. To the present one dimen- 
sional approximation, the current in the medium at 
.V and time ; is given by — /i(.v + vt'). Denoting the 



^1 



Xz 



_fL 



'<'^(«?<^{M(( (^</fulff:::::ffffrkh 



^ ^ / / /y /^//. 



y. 



X| X 

V77\ 



//////////A 



X2 



!>i>;'Hi>!>^!>;'; 



//^^//// 



JL 



222 




FIG. 25. A. A uniform axon immersed in a conducting Huid 
medium of uniform diameter; the action potential recorded 
with electrodes at xi and x-; is given by the equation (9-3). 

B. The case in which the diameter of the fluid medium changes 
at x'; the action potential recorded is given by equation (9-4). 

C. A uniform axon immersed in a large volume of fluid; the 
potential near the axon is given by the triphasic curve in the 
diagram. 



potential at .vo in the medium referred to that at .vi by 
U2-1, it is found that 



- = L 



si, fix + I'Odx 



VQx, + vt) V(,Xi + vt}. 



(9-3) 



[Note that the integral above represents a summation 
of the IR drops along the fluid medium at a given 
moment /.] The action potential recorded externally 
with electrodes placed at Xi and X2, U2-1, consists of 
two terms, one representing the activity at Xi, 
F(Ar2 -f vt), and the other, the activity at xi, F(xi + 
vt). The amplitude of the observed potential variation 
is reduced by a factor of j/n- Equation (9-3) is a 
mathematical expression of what is known as 'diphasic 
recording' of the action potential. Because of the 
negative sign in front of VQx^ -\- vt), it was believed 
that the surface of the active portion of an axon was 
'electrically negative' to the surface at rest. It should 
be borne in mind, however, that, if the surrounding 
inedium is not uniform, the potential on the active 
surface is not always negative to that on the resting 
surface. 

The next simple example of the volume conductor 
problems is the case in which the resistance per unit 
length of the conducting fluid medium changes at x' 
suddenly from si to so (fig. 25Z?). Expressing j- as a 
function of ,v, it is found that 



U,^i 



-f 



<.v)/,(.v -I- vt) d.v 



- 1 n , - SV(^x -I- ;./) ^ 



dx 



(9-4) 



_5 K(.v, + ,-0 - — ' F(.v> + vt) 



+ 



V\x' -I- vt). 



[The last step of the calculation above was accom- 
plished by integration by parts.] The right-hand mem- 
ber of equation (9-4) contains three terms, the first 
term representing the activity at x-t, the second term 
that at .V] and the third term arising from the activity 
at .v'. The third term changes its sign, depending on 
whether s-i < s\ or .f-.> > S\;\X vanishes when si = 5\. 
If Si is nearly zero, i.e. if the amount of fluid around 
the axon is very large on one side of x' , the second 
term in equation (9-4) vanishes and the equation in- 
dicates that the electrode at .vi effectively records the 
potential variation at x' . 



io6 



HANDBOOK OF PHVSIOLOGV 



NEUROPHYSIOLOGY 1 



The final case to be discussed is the potential field 
produced by a uniform axon suspended in a large 
volume of conducting fluid. In this case, the potential 
in the fluid at a great distance away from the axon is 
not influenced by the nerve impulse; therefore, the 
electrode on such a point is truly "indifferent'. Under 
such circumstances, the potential in the space is in- 
vensely proportional to the distance from the source 
of current. Since there is a line source in the present 
case, the potential at point P in the fluid medium is 
given by 



f/. 



4^ J 






d.v, 



where .S' is the specific resistance of the fluid medium 
and /?,,(.v) is the distance between point P and point x. 
If point P is on the surface of the axon (at x = p), 

U,. cc /,„(/) + ,.t\ 

since the source in the immediate neighborhood ol the 
recording electrode is expected to have an over- 
whelmingly large effect in determining U,,. The time 
course of Up is now triphasic as is the time course of 
the membrane current in figures 23 and 24. Under 
these circumstances it is incorrect to say that the sur- 
face of the active region of the axon is 'electrically 
negative'. 

More complicated cases of the volume conductor 
problems can be solved by finding the solution of 
Laplace's equation AT' = o under the boundary con- 
dition described roughly by ( — i /.S) (3 r/c)«) = 
/,„(.v + r/), where n is the normal to the surface of the 
axon. To apply this concept of volume conductors to 
the potential field in the body, one has to consider 
both the nonuniformity of the excitable tissues and 
the nonhomogeneity of the conducting medium. The 
arguments described above on the potential field 
cau.sed by nerve impulses are based on the work of 
Craib (21), Marmont (83), Lorente de No (77), 
Tasaki & Takeuchi (136) and others. 



NERVOUS CONDUCTION IN MYELINATED NERVE 
FIBER (SALT.ATORY CONDUCTION) 

The mode of propagation of a nerve impulse in 
the vertebrate myelinated nerve fil)er is expected to 
be somewhat diflferent from that in the invertebrate 
nerve fiber because of the structural discontinuities 
along the myelinated nerve fiber. We have seen that 
the myelin sheath of the vertebrate nerve fiber shows 
an cxtremelv hia;h electric resistance to a direct cur- 



rent (p. 87). We have also become acquainted with 
the experimental evidence indicating that the elec- 
tric response of the nerve fiber derives from physio- 
logical activity localized at nodes of Ranvier of the 
fiber (p. 88). The myelinated nerve fiber has a 
cable structure; when one of the nodes of the fiber is 
thrown into action, there is a local current which 
tends to raise the membrane potential of the adjacent 
node to a level higher than the threshold potential. 
When all the nodes of the fiber are excitable, there- 
fore, it is expected that the activity will spread from 
node to node indefinitely along the fiber. We shall 
examine the line of evidence indicating that this is 
actually the mode of nervous conduction in the 
mvlinated nerve fiber. 



Effect of Increase of External Resistance 

It is a fairly difficult problem to demonstrate that 
an increase in the resistance of the external fluid 
medium does affect propagation of a nerve impulse 
in the myelinated nerve fiber. The reason is that the 
resistance per unit length of the axis cylinder is very 
high (150 to 250 Mfl per cm) even in the largest 
nerve fiber in the frog .sciatic nerve. Unless the ex- 
ternal resistance is raised above this level of the in- 
ternal resistance, it would not be possible to demon- 
strate a clear effect upon the process of nervous 
conduction. 

The first piece of evidence along this line was ob- 
tained in the nerve fiber of which a portion was 
rendered inexcitable by narcosis (117, 135). The 
upper part of figure 26 shows the experimental 
arrangement employed. An isolated nerve fiber of 
the toad is mounted across three pools of Ringer's 
fluid separated by two narrow air-gap partitions. A 
portion of the fiijer, including two nodes of Ranvier, 
is introduced into the small middle pool, and the 
remaining portions of the fiber are immersed in the 
large lateral pools. In each of the three pools, an 
electrode of Ag-AgCl Ringer (agar) type is im- 
mersed. The electrode in one of the lateral pools is 
connected to a low input amplifier, and the remaining 
two electrodes are grounded. 

With all three pools filled with normal Ringer's 
solution, the nerve impulse arising at E in the figure 
alwavs travels across the two narrow partitions (record 
A). When the portion of the fiber in the middle pool 
is treated with a cocaine-Ringer's solution (0.2 per 
cent), the impulse fails in some preparations to 
propagate beyond the narcotized region (record B). 
When the electrode in the small middle pool is 



CONDUCTION OF THE NERVE IMPULSE 1 07 

Fig. 28 



y AMP. 




'MX^^ 






Fig.26 




v-r>. 




FIG. 26. Demonstration of the dependence of ncivous conduction upon the flow of electric current 
outside the fiber. A. Action current recorded with an amphfier connected between the middle and 
the distal pools; stimulus given at E. B. Block of conduction caused by replacing the Huid in the 
middle pool with an 0.2 per cent cocaine-Ringer's solution. C. Restoration of conduction by lifting 
the middle electrode from the surface of the fluid. Time marks, i msec, apart. [From Tasaki (123).] 

FIG. 27. Demonstration of the effect of a shunting resistance of 20 megohms across the insulated 
internode upon nervous conduction. AMP represents a high input-impedance preamplifier. Record 
A was taken with the resistance disconnected; Record B with the resistor connected. [From Tasaki & 
Frank (128).] 

FIG. 28. Measurement of the safety factor in nervous conduction by narcosis. Top record: Normal 
binodal action current. Second through Joiirth records: 3, 7, 38 and 38.1 minutes after introduction of a 3 
per cent urethane-Ringer's solution into the proximal pool. [From Tasaki (124).] 



lifted above the surface of the saline at this moment, 
there occurs a marked increase in the recorded cur- 
rent and, at the same time, the tiine course of the 
current becomes diphasic (record C). In a motor 
nerve fiber with its innervating; muscle left intact, it 
is seen that the diphasicity in the recorded current is 
always associated with propagation of an impulse 
across the narcotized region in the middle pool. 

The mechanism of restoration of conduction in 
this experiment is as follows. The portion of the fiber 
in the middle pool treated with cocaine is inexcitable. 
The activity of the portion of the fiber in the lateral 
pool induces a current that spreads along the fiber in 
the middle pool, but this spreading current is too 



weak to e.xcite the portion of the fiber beyond the 
middle pool. When the electrode in the middle pool 
is removed, the leakage of the spreading current 
through the portion of the fiber in the middle pool 
is reduced and, consequently, the current that reaches 
the other side of the middle pool is increased. Thus, 
the spreading current becomes suprathreshold for 
the portion of the fiber beyond the middle pool. 

The question has been raised (37, 66, 128, 145) as 
to whether it is possible to block propagation of a 
nerve impulse by insulating a nerve fiber between 
the two neighboring nodes. First, we must discuss a 
troublesome factor related to the experiment de- 
signed to answer this question. 



io8 



HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



In order to detect propagation of ner\e impulses 
across an insulating air gap, it is necessary to have 
an amplifier or the innervated muscle attached to 
the single fiber preparation. Stimulating electrodes 
and a muscle or recording electrodes connected to 
the two sides of the insulating gap introduced an 
electric capacity which, under ordinary experimental 
conditions, is large enough to establish a local circuit 
(by this capacitativc pathway). The resistance of a 
single fiber preparation mounted across a wide air 
gap is of the order of 50 MfJ. If there is a capacity of 
about 2 MMf between the two portions across the gap, 
the local circuit between the two portions of the 
preparation will be very effectively closed by the 
capacitative pathway for a period of about o. i msec. 
In fresh single fiber preparations, it is actually im- 
possible to demonstrate a conduction block at an 
insulating air gap if muscular contractions are taken 
as an index of such conduction'' (128, 145). 

The capacitative coupling between the two insu- 
lated portions of a single fiber preparation can be 
markedly reduced by the use of a positive feed-back 
amplifier. In the diagram of figure 27 the small por- 
tion of the preparation on one side of the insulating 
air gap is connected to the input of a unity-gain 
preamplifier and is completely enclosed in a metallic 
shield driven by the output of the preamplifier. (Note 
that, when the potential of the insulated portion 
rises above the ground potential, the potential of the 
shield around the fiber rises to the same extent and, 
consequently, no electric charge is induced between 
the insulated portion of the preparation and ground. 
The input impedance of the preamplifier can l)e 
made as high as 1000 MQ.) 

It is surprising to see that most single-fiber prepa- 
rations mounted as shown in this figure are still 
capable of carrying impuLses across the air gap (128). 
Washing the surface of the internode in the gap with 
a nonelectrolyte solution does not generally help to 
bring about a block at the insulating air gap. Prob- 
ably, the cell of Schwann on the surface of the nerve 
fiber does not permit us to raise the external resist- 

' There are somewhat controversial viewpoints on this 
subject in the literature. Huxley & Stampfli (66) reported 
that conduction was blocked when the external resistance was 
raised. Wolfgram & van Harreveld (145) failed to demonstrate 
a block under similar experimental conditions and expressed 
the view that their experimental results were inconsistent 
with the concept of saltatory conduction. Frankenhauser & 
Schneider (37) reported that they could demonstrate a block 
with a 20 MSJ shunting resistance across the insulating air 
gap. For a further discussion on this point, see Tasaki & Frank 
(128). 



ance high enough to cause a conduction block in 
fresh preparations. 

Record .-1 in figure 27 was obtained after circulat- 
ing dry air around the portion of the fiber on the air 
gap for a short period of time. This causes a rapid 
e\aporation of water from the surface of the fiber 
followed by a slow desiccation of the axis cylinder. 
The monophasicity of the response indicates that the 
block has actually taken place. Record B in the 
figure was taken while the small insulated portion 
of the preparation was grounded through the 20 M12 
resistor in the figure. The response is now diphasic 
(or rather binodal), indicating that conduction was 
restored by the shunting resistance. A similar revers- 
ible restoration of conduction can be obtained by re- 
ducing the feed-i)ack voltage to the driven shield, 
thereijy increasing the capacity of the insulated por- 
tion of the preparation to ground. 

The obsersation just described indicates that the 
abilit\- of the ner\e impulse to excite the adjacent 
resting region is \cry large. As a consequence, a re- 
versible conduction l:)lock by increasing the external 
resistance has been demonstrated so far in prepara- 
tions with a somewhat reduced safety margin. How- 
ever, it seems safe to conclude from the observations 
described above that ner\-ous conduction in the 
myelinated nerve fiber does depend on the electric 
pathway outside the myelin sheath. 

Safety Factor 

The safety factor in ner\ous conduction inay be 
defined as the ratio of the action current of the nerve 
fiber to the minimum current intensit\ necessary for 
ner\ous condtiction. If an action current generated 
at one point of the nerve fiber acts as an electric 
stimulus to the adjacent point, it should be po.ssible 
to measure the action current in terms of the normal 
threshold. 

The first attempt to determine the safety factor was 
made bv using a dilute narcotic solution to reduce 
the action current from one portion of a nerve fiber 
and by measuring the minimum intensity of the cur- 
rent necessary to excite the adjacent portion of the 
fiber (135). In the uppermost part of figure 28 is 
shown the experimental setup used. A motor nerve 
fiber of the toad is mounted across two pools of 
Ringer's fluid separated by a narrow air gap. The 
muscle innervated by the fiber is left uncut, and 
twitches in the muscle resulting from stimulation of 
the fiber near its proximal end are taken as an index 
of nervous conduction. An ohmic resistor (of about 



CONDUCTION OF THE NERVE IMPULSE 



109 



0.2 M12) is connected between the two electrodes im- 
mersed in the pools, this resistor serves to close the 
external pathway of the local circuit and also to 
measure the lons^itudinal current flowint^ through the 
axis cylinder bridging the air gap. 

When the two pools are filled with normal Ringer's 
solution, a familiar action current which we often 
refer to as a ' binodal' action current is recorded. 
Based upon the arguments described on earlier pages 
(p. 88), this action current is explained as deriving 
mainly from activity at the nodes (Ni and No in the 
figure) in the immediate neighborhood of the record- 
ing partition. The rapid rising phase of the action 
potential at Ni develops a large gradient of potential 
along the axis cylinder between node Ni and N2; the 
phase of a strong (2 to 3 times lo"" amp.) current 
flow in the binodal action current is the period during 
which Ni is active but N2 is still inactive. When the 
action potential starts also at node N>, the potential 
gradient along the axis cylinder is greatly diminished, 
resulting in a sudden fall in the longitudinal current 
between Ni and N-j. At the end of the action potential 
of a single node (fig. 16), the membrane potential 
falls very rapidly. The abrupt end in the binodal 
action current is related to the difference in the time 
of termination of the action potential at Ni and N2. 
Because of the capacities of the nodal membrane and 
of the myelin sheath, the spread of current from No 
to the internode between Ni and N2 prior to the 
start of activity at Ni is very small. 

When a urethane-Ringer's solution barely strong 
enough to block nervous conduction is introduced 
into the proximal pool (in which Nj is immersed), the 
upward deflection in the record (representing posi- 
tivity of the right-hand electrode in the diagrain) 
gradually decreases, indicating that the current arising 
at Ni (partly from No) is reduced b)' narcosis. When 
the upward deflection is reduced to one-fifth to one- 
seventh of the original size, the downward deflection 
which has gradually increased during narcosis sud- 
denly drops out and, simultaneously, conduction 
across the recording internode fails (the lowermost 
record in fig. 28). From these observations, it is 
found that the safety factor is between fi\e and seven 
in large myelinated nerve fibers of the toad. 

The safety factor can be estimated from the meas- 
urement of the threshold membrane potential and 
the nodal action potential. It has been shown that 
the action potential of a normal node is approximately 
1 10 mv at the peak. When a membrane potential of 
this size is developed at node Ni, the adjacent node 
N2 is subjected to a strong outward current which 



would raise the membrane potential by 50 to 60 mv 
if N2 had been made ine.xcitable (124). Since the 
threshold depolarization of a fresh node is 10 to 15 
mv, it is found that the safety factor estimated by this 
method is about five. There are other methods of 
estimating the safety factor (124). They all give a 
figure between four and seven. 

As the result of this large safety factor in nervous 
conduction, a nerve impulse can travel across one or 
sometimes two completely narcotized nodes (124). 
In the experiment of figure 26 it is often seen that 
conduction across the middle pool remains unsus- 
pended after introduction of a strong narcotic solu- 
tion. A nerve impulse cannot travel across three 
inexcitable nodes. 



Dots the .\ervc Imjnihc Jiiinji Jrom A'odr to Node? 

In 1925 Lillie (75) found that, when his iron wire 
model of a nerve was covered with glass tubing broken 
at regular intervals, the activation process jumped 
from one break to the next. On the basis of this ob- 
servation, he pointed out the possibility that the 
nerve impul.se in the myelinated nerve fiber may 
jump from node to node as in the model. This model 
of 'saltatory conduction' has the following two fea- 
tures: (a) the electrochemical changes underlying the 
process of 'conduction' are localized at the 'nodes' 
and (i) the time required for the conduction of the 
impulse is determined solely by the rapidity of the 
process at the node. In the model, therefore, the role 
of the internodal segment is simply to provide an 
ohmic conductance to the local circuit. 

We have described the main line of evidence indi- 
cating that, in the vertebrate myelinated nerve fiber, 
the physiological process responsible for producing 
action potentials is localized at the nodes. We have 
also seen that, although the d.c. resistance of the 
myelin sheath is very high, the capacity of the myelin 
sheath is large enough to have a marked effect upon 
the threshold of the nerve fiber measured with short 
current pulses (p. 99). This capacity of the myelin 
sheath, therefore, sets a certain limitation to the 
analogy between propagation of the activation wave 
in the iron-wire model and the actual process of 
ner\'ous conduction in the mvelinated nerse filjcr. 

The upper part of figure 29.I illustrates the arrange- 
ment to demonstrate saltatory conduction in the 
model nerve fiber. An iron wire covered \vith glass 
tubings except at the ' nodes' is immersed in a bath 
of nitric acid. When the wire in the passive state is 
stimulated at one end, the process of activation as 



HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



B 



^/ //////////////J^''//S^/////, j-LL . 



'////'' 



v^y-ri^ 




Iz 






I I a/. 




r -► 




4 Ql2 mite 



FIG. 29. A. Time courses of the longitudinal current at two points in one internode of Lillie's salta- 
tory nerve model. [From Franck (35).] B. Time courses of the longitudinal current recorded at two 
extreme ends in one internode of a frog nerve fiber. Stimulus at E. [From Hodler ei al. (64).] 



recognized by color changes and bubbling on the 
surface spreads from node to node. The trace repro- 
duced in the figure is the time course of the longitudi- 
nal current taken from a recent article by Franck (35). 
Since the glass tubing is a perfect insulator of elec- 
tricity, the time courses of the longitudinal currents 
recorded at two different points in one internode are 
undoubtedly the same. 

Figure 29^ shows a corresponding observation on 
the real myelinated nerve fiber. By the arrangement 
illustrated at the top, the longitudinal current is re- 
corded at two points in one internodal segment. As 
can be seen in the tracing below, there is a large dif- 
ference between the longitudinal currents recorded 
at two points which are about 1.5 mm apart in this 
case. The difference between the two longitudinal 
currents represents the double peaked membrane cur- 
rent recorded through the myelin sheath (fig. 11 A). 

We see in figure 2(jB that the two longitudinal 
currents recorded at two different points in one 
internode rise at different rates, reach the peaks at 
different moments and fall at different rates. This is 
a direct consequence of the existence of a large capaci- 
tative flow of current through the myelin sheath. 
Like a signal travelling along a submarine cable, the 
longitudinal current spreads along the axis cylinder 
at a finite rate.'" Because of this slow spread of the 
membrane potential (cf. p. 100) and of the longi- 

'" A different viewpoint is stated in a previous paper by 
Huxley & Stampfii (66). The slight difference between 
their experimental results and the results described in the text 
is probably due to their use of a high input resistance in their 
amplifier which tends to lower the time resolution in recording 
[cf. footnote on p. 11, Tasaki (124)]. 



tudinal current along the internode, it is not legiti- 
mate to state that a nerve impulse jumps from node 
to node without spending any time in the internode. 
This point has been stressed in an article by Hodler 
et al. (64) [cf. also Stampfli (i 14)]. It has been pointed 
out (125) that the major portion of the temperature 
dependence of the conduction velocity (Qio of about 
1.8) can ije attributed mainly to a change in the 
cable properties of the nerve fiber [cf. also Schmitt 
C106)]. 

Field of Piitential Produced by a .\enr Impulse 

We have discussed in the preceding section the 
field of potential produced in the surrounding fluid 
medium by a nerve impulse travelling along a uni- 
form invertebrate nerve fiber. Because of the struc- 
tural discontinuities along the myelinated nerve 
fiber, the statements made in the preceding section 
are not in a strict sense applicable to the myelinated 
nerve fiber. However, there is a special case in which 
the effect of the discontinuities is very small. 

Let us consider the case in which a single nerve 
fiber of a uniform diameter is enclosed in a glass 
tubing of a uniform diameter filled with Ringer's 
.solution (as in fig. 25.-1). In this ca.se, the longitudinal 
current at one point along the fiber is equal in in- 
tensity and opposite in sign to the current flowing 
through the medium at the same point. From the 
argument described in the preceding section, it is 
found that the spatial distribution of the potential 
along the fluid medium in the glass tubing is a mir- 
ror image of the potential inside the axis cylinder, 
its absolute value being determined by the ratio of 



CONDUCTION OF THE NERVE IMPULSE I I I 



the resistance per unit length of tlie outside fluid to 
that of the axis cylinder (equation 9-3). This field of 
potential travels along the fiber at the average ve- 
locity of the impulse. Insofar as one disregards the 
variations in the potential that occur within one 
internodal distance (about 2 mm) or within one 
internodal conduction time (about o. i msec), the 
potential field produced by a myelinated nerve fiber 
in the fluid medium is similar to that produced by a 
uniform invertebrate axon. 

The distribution of the potential on the surface of 
a uniform nerve trunk produced by a nerve impulse 
travelling along a single nerve fiber in the trunk can 
be regarded as analogous to the case described above. 
To the approximation that the potential variations 
within 0.1 msec, are disregarded, therefore, the 
principle of 'diphasic recording of the action poten- 
tial' described in the preceding section is applicable 
to this case. A further discussion on this problem can 
be found elsewhere (124). Frankenhauser (36), 
Hodler el al. (64), Stampfli & Zotterman (i 15) and 
others have investigated the details of the potential 
variations occurring within one internodal conduction 
time and also within one internodal length. 

When a myelinated nerve fiber is immersed in a 
two-dimensional or three-dimensional volume con- 
ductor, the potential field produced ijy a nerve im- 
pulse is very different from the field produced by an 
impulse of a uniform invertebrate axon. As has been 
shown in figure 1 1, strong sinks of electric current are 
localized at the nodes while the sources are distributed 
along the internodes as well as at the nodes. There- 
fore, the time course of the potential picked up by a 
recording electrode placed near one of the nodes is 
expected to be very different from the record ob- 
tained with the electrode on the myelin co\ered por- 
tion of the fiber. 

Figure 30 shows the time courses of the action 
potentials recorded with a metal microelectrode 
placed at various points near a node of Ranvier of 
an isolated single nerve fiber immersed in a thin 
layer of Ringer's solution. The vertical straight line 
in the middle of the figure represents the course of 
the fiber, and the center of the two concentric circles 
represents the position of the node under study. It is 
seen in the figure that the largest negative potential 
is observed when the recording electrode is placed in 
the immediate neighborhood of the node. The ampli- 
tude of the negative component of the action poten- 
tial decreases as the distance from the node increases, 
and this decrease is roughly independent of the direc- 
tion in which the electrode is moved awav from the 




FIG. 30. Records of action potentials taken with a small 
metal electrode placed around a node of Ranvier. The nerve 
fiber was immersed in a shallow layer of Ringer's solution. 
The vertical line represents the nerve fiber, and the center 
of the two concentric circles the node under study. The im- 
pulse travels downward. Five records on the vertical line were 
taken with the electrode along the fiber and slightly to one side. 
Other nodes of the fiber were not exposed in the operated 
region of the preparation. The conduction distance was about 
45 mm. Temperature, 2o°C. [From Tasaki (137).! 



fiber. For further details of this experiment see Tasaki 

(124). 

Conduct uin in a Polarized Nerve Fiber 

When a direct current is applied to a nerve trunk 
through a pair of nonpolarizable electrodes in con- 
tact with its surface, the portion of the nerve fiber 
near the anode is traversed by a continuous inward 
membrane current, and the region near the cathode 
is subjected to an outward membrane current. The 
behavior of the nerve impulse in such ' polarized' 
regions of the nerve fiber was discussed by Pfliiger 
(too) more than a half century ago. A nerve fiber 
modified by a constant current is said to be in an 
'electrotonic' state. In order to understand the be- 
havior of a nerxe impulse in the nerve fiber under 
'electrotonus', it is desirable to investigate the be- 
havior of a single node preparation under influence of 
a constant current. 

Figure 31 shows the effect of a passage of a short 
rectangular current pulse upon the threshold and the 
action potential of the single node. The arrangement 
employed is the same as that for figure 16 (p. 94). 



HANDBOOK OF PH^■SK5I,0^,Y 



NEUROPHYSIOLOGY I 




FIG. 3 1 . Effect of short polarizing current pulses upon the 
action potential of a single node of Ranvier. The arrangement 
shown in the upper part of fig. 16 was used. The action poten- 
tial was initiated by a short stimulating pulse approximately i 
Qejl) and 4 msec. (jighQ after the start of the polarizing pulse. 
Voltage calibration, 50 mv; time marks, i msec. .\ toad nerve 
liber at 1 1 °C. [From Tasaki (126).] 



When a pulse of outward subthreshold current is 
applied through the nodal membrane, the potential 
inside the node rises above the resting level, resulting 
in an upward deflection in the record. The threshold 
membrane potential measured during the period of 
current flow (of about 10 msec.) is nearly identical 
with the level before the start of the subthreshold 
pulse. In other words, a weak additional current, 
which is sufficient to raise the membrane potential 
from the new level reached by application of the sub- 
threshold pulse to the normal threshold level, re- 
leases a full-sized action potential. The membrane 
potential at the peak of the action potential is also 
unaff"ected by the constant current. 

When the polarity of the constant current is re- 
versed, a stronger additional stimulating pulse is re- 
quired to raise the membrane potential to the 
threshold level. The membrane potential at the peak 
of the action potential is not affected by application 
of a constant inward current of about 10 msec, 
duration. 

In the experiment just described, if one regards the 
threshold for the short (additional) current pulse as a 
function of the rectangular polarizing current, one 
finds that the threshold is lowered by an outward 
polarizing current and raised by a current of opposite 
polarity. Similarly, if one measures action potentials 
from the level immediately before the delivery of the 
short stimulating pulse, it is found that the amplitude 
is reduced by an outward (or cathodally polarizing) 
current and increased iiy an inward (or anodally 
polarizing) current. This is the direct, or primary 
eff'ect of the polarizing current upon the threshold 
and the action potential. 

A long polarizing current brings about a secondary 
change in the membrane. A strong maintained 



cathodal polarization caused an additional decrease 
in the amplitude of the action potential (cathodal 
depression) accompanied by changes in the mem- 
brane conductance and probably in its emf The 
effect of a strong anodal polarization is somewhat ob- 
scured ijy the strong stimulating current required to 
raise the membrane potential up to the threshold 
level. Using intact sciatic nerves, Lorente de No (77) 
made an extensive investigation on the changes in 
the membrane potential caused by long polarizing 
currents. 

Now let us disctiss in this connection the well-known 
experiment iiy Erlanger & Blair (28) who in 1934 
discovered the electric sign of the discontinuous na- 
ture of nervous conduction in the myelinated nerve 
fiber. They applied anodal polarization to the por- 
tion of the nerve under the recording electrode (mono- 
phasic lead) and found that, when the intensity of 
the polarizing current was gradually increased, the 
configuration of the action potential of a single nerve 
fiber in the nerve underwent a sudden discontinuous 
change. Figure 32 furnishes an example of their 
record. In record B the intensity of the polarizing 
current was maintained at the critical level for the 
discontinuous change. The action potential showed 
in one sweep a distinct notch in its rising phase, and 
in the next .sweep (superposed on the same film) 




FIG. 32. Changes in the configuration of a monophasic 
action potential of a single nerve fiber in an intact nerve trunk 
produced by anodal polarization at the proximal recording 
lead. .-1. The normal spike potential. B. The spike under anodal 
polarization just strong enough to block at the most accessible 
node; two action potentials superposed. C. Further increase 
in the polarizing current to the next critical strength. [From 
Erlanger & Blair (28).] 



CONDUCTION OF THE NERVE IMPULSE 



I'3 



the component of the action potential above the 
notch dropped out completely. 

They did not consider this observation as indicating 
the saltatory nature of nervous conduction in the 
myelinated nerve fiber. However, they correctly ex- 
plained this discontinuity as being related to the 
existence of nodes along the myelinated nerve fiber. 
Takeuchi & Tasaki (nS) repeated this observation 
on isolated single nerve fibers and obtained substan- 
tially the same result. 

The explanation of the discontinuous change in 
the single fiber respon.se (fig. 32) is as follows. When 
the threshold membrane current of the anodaliy 
polarized node under the recording electrode rises 
above the membrane current caused by the activity 
of the adjacent node, the response of the node under 
study drops out and a small potential variation arising 
from the activity of the adjacent node is observed. 
A further discussion on this subject may be found 
elsewhere (124). 

Pfliign' s Law of Contraction 

The law of contraction formulated by Pfli'iger (loo) 
in 1859 is at present of almost historical interest only. 
To demonstrate this law one has to use a pair of non- 
polarizable electrodes, e.g. long chlorided silver wires 
imbedded in 2 per cent agar-Ringer's gel filled in 
glass tubings or classical electrodes of the Zn-ZnS04 
type. A sciatic-gastrocnemius preparation of the frog 
or toad is the standard material used for this demon- 
stration. When pulses of constant current (of about 
10 sec. duration) are applied to the nerve trunk 
through the nonpolarizable electrodes, one generally 
finds that contractions of the muscle, if there are 
any, occur only immediately following the onset or 
following the end of the pulse but not during the 
period of constant current flow. The presence or ab- 
sence of contraction depends upon the intensity of 
the current and also upon the arrangement of the 
anode and the cathode of the stimulating electrodes 
with respect to the muscle. In table 2 an example is 
presented of the results of this type of observation. 
The symbol -|- indicates the presence and — the 
absence of a muscular contraction. The appearance 
of a contraction is a sign of arrival of nerve impulses 
in the muscle. 

If one takes nerve impulses carried to the muscle 
by a single nerve fiber as an index, one obtains a 
result somewhat different from that stipulated by 
the classical law. The result obtained after cutting 
all but one fiber near the mu.scle is also shown in 



T.\BLE 2. Demonstration of Pfliiger' s Law of Contraction 



Current 


Cathode- 


Anode-Muscle 


Anode. Cath 


ode-Muscle 


Intensity 


(Ascending) 


(Descending) 


wA 


Make 


Break 


Make 


Break 


4.5 


+ (-) 


- 


+ (-) 


— 


6 


-1- 


— 


+ 


— 


18 


-f- 


— 


-f 


— 


30 


+ 


— 


+ 


— 


.52 


+ 


-f-(±) 


-f- 


+ C-) 


75 


+ C-) 


+ (±) 


-1- 


+(-) 


98 


+(-) 


+ 


-1- 


+ C±) 


120 


+(.-') 


+ (±) 


+ 


+(±) 


144 


+(-) 


+ C±) 


+ 


+(-) 


166 


_ 


+(±) 


+ 


±(-) 


188 


— 


+(-) 


+ 


— 



This table indicates the presence, 4-, or the absence, — , 
of a muscular contraction on make or break of long current 
pulses applied to the nerve trunk of a sciatic-gastrocnemius 
preparation of the toad. The orifice of the electrodes (Ag- 
AgCl type) was about 6 mm in diameter and the space be- 
tween the two electrodes was also about 6 mm. The resistance 
of the nerve between electrodes was approximately 10 
kilohms. The results obtained after cutting all the nerve fibers 
near the muscle except one large motor fiber are presented 
in parenthesis, and is mentioned only when it is different 
from that for the whole nerve preparation. 



table 2. There are more negative signs in this case 
than in the case for the whole nerve trunk. This 
difference arises from the situation that there are in 
the nerve trunk many fibers which are situated in 
different parts of the potential field (produced by the 
applied current). The existence of the small motor 
nerve fibers which produce slow muscular contrac- 
tions (134) in the nerve trunk makes also some dif- 
ference between a single fiber and a nerve trunk ex- 
periment. 

If one applies current pulses directly to the iso- 
lated portion of a single motor nerve fiber in this 
type of observation, one finds more negative signs 
than in the two previous cases. In this type of direct 
stimulation of a single nerve fiber, it is very difficult 
to demonstrate excitation of the fiber on break of an 
applied current. Break excitation which is readily 
observable in the nerve trunk is evidently due mainly 
to the capacities of the myelin sheath and of the con- 
nective tissues. These elements in the nerve trunk 
tend to generate outward membrane currents at the 
nodes of the fibers on withdrawal of the applied cur- 
rent. 

The mechanism of anodal block of nerve conduc- 
tion has been discussed on previous pages. The ab- 



114 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



senceof a contraction on" make' of a strong' ascending' 
current pulse in the table indicates that the nerve 
impulse initiated under the cathode could not pass 
through the anodally polarized region of the nerve 
between the cathode and the muscle. 

Effect of Narcosis upon Nervous Conduction 

It has Ijeen pointed out that narcotics, such as 
cocaine, urethane, ethanol and others, depress or 
eliminate electric responses of the nerve fiber when 
they are applied to the nodes of Ranvier of the fiber 
(p. 109). The action of these chemicals, as well as 
the effect of low sodium in the medium, progresses 
with surprising rapidity; an equilibrium between the 
single fiber and the surrounding fluid medium con- 
taining these chemicals is established within one 
second (68, 71, 124). 

It is well known that the action of these narcotics 
upon the whole nerve trunk is extremely slow and 
gradual, as emphasized by Winterstein (144). Evi- 
dently the time required for dififusion of the chemical 
into the nerve trunk accounts for the slow action of 
these chemicals upon it. The diagram in figure 33 
illustrates this great difference in the rapidity of ac- 
tion of urethane between the intact sciatic nerve and 



O min 
O 

_J 
CD 

-:»40 



I- 
O 

q30 

Z 

o 
o 

tr 
020 

b. 

Q 
UJ 

cc. 

510 

o 

LJ 
CC 

UJ 



', SCIATIC 
' NERVE 



o 

9 



e> 
© 
o 

\ 

o 



^8. § 



SINGLE 

NERVE FIBER 



o ~« 
o o 



I 



20 2 4 6 8% 10 

^ URETHANE RINGER 

FIG. 33. Relation between the concentration of urethane 
(abscissae) and the time required for conduction block (ordi- 
nates') in a single fiber preparation (continuous line) and in the 
whole sciatic nerve (circles'). [From Tsukagoshi, cited by Tasaki 
in (124).] 



the exposed nerve fibers. The circles in the figure 
represent the times required for conduction block at 
various concentrations of urethane-Ringer's solution 
in intact sciatic nerves. The narcotic was applied to 
a 1 5 mm long uniform portion of the sciatic nerve 
of the toad, and the disappearance of muscular con- 
traction was taken as an index of block. The relation- 
ship between the time required for conduction block 
and the concentration of urethane is given by a 
smooth curve. 

When a single nerve fiber preparation or a few 
fiber preparations with a 15 mm long exposed region 
are used in this type of e.xperiment, an entirely dif- 
ferent result is obtained. For concentrations lower 
than about i .8 per cent, conduction through the 
narcotized region remains unblocked for an hour or 
more; and for concentrations higher than about 2.2 
per cent, conduction block sets in within one second. 
At the critical concentration, which was appro.xi- 
mately 2 per cent in this experiment, conduction 
block occurs within about i minute. The relationship 
between the concentration and the time required for 
blocking is, therefore, given by the thick line (bending 
at almost a right angle) in the figure. 

We have pointed out that a nerve impulse can 
jump across one or two complete inexcitable nodes. 
If one reduces the length of the narcotized region 
down to about 5 mm or less, a longer time is required 
to block conduction since time must be allowed for 
diffusion of the narcotic along the nerve. This fact 
was once taken as evidence for ' decremental conduc- 
tion' in the narcotized region of the nerve (23, 69, 
82). 

Narcotizing solutions of below the critical concen- 
tration applied to a node raise the threshold and lower 
the amplitude of the response. The magnitude of this 
narcotizing effect depends on the concentration used. 
Based on the experimental data on the effects of 
narcosis on single nodes, it is possible to explain 
nianv phenomena related to narcosis of the nerve. 
The detail of the accounts along this line may be 
found elsewhere (124). 



.\FTER-POTENTL^LS .^iND RHYTHMICAL .\CTIVITY 

We shall devote this section to two subjects which 
are less clearly understood at present than tho.se 
previously discussed, namely after-potentials and 
rhythmical activity of the nerve fiber. The relation- 
ship between after-potentials and rhythmical activity 



CONDCCTION OF THE NERVE IMPULSE 



is not a direct one, but in some cases they are clearly 
related to each other. 

After-Piilentials 

The term ' after-potential' was introduced by Gasser 
and his associates [cf. Gasser & Erlanger (38); Gas- 
ser & Graham (39)] to describe the small, slowly 
declinina; potential change that follows the large, 
short 'spike-potential' in the monophasic action po- 
tential of a nerve trunk. The records furnished in the 
left column of figure 34 show monophasic action 
potentials of the nerve trunk taken at slow sweep 
speeds. The action potential of A-fibers (top) shows 
very little after-potentials, but the responses of B- 
and C-fibers manifest large after-potentials following 
the sharp spike-potentials. These potentials were re- 
corded with extracellular electrodes (fig. i) from three 
different nerve trunks of the cat. 

Let us ne.xi discuss the after-potentials recorded 
from single fiber preparations. In the right-hand 
column of figure 34 are shown the time courses of 
the action potentials of three different kinds of e.\- 
citable elements. An upward deflection in these 
records represents a rise in the intracellular potential 
(referred to the potential of the surrounding fluid 
medium). The 'retention' of a higher potential level 



Mf 



I... I.. 



.I...I...I. 



Tiiiec 



50 
msec 




200 msec 



FIG. 34. After-potentials in nerve trunks Qejl) and in single 
fibers (right). A. Response of mammalian .-X-fibers. B. Re- 
sponse of mammalian B-fibers. C. Response of mammalian 
C-fibers. [The three records on the left are from Grundfest 
(43)-] ^J' Action potential of a toad muscle fiber, recorded 
intracellularly. Nf. Response of a toad nerve fiber poisoned 
with veratrine. Sf. .Action potential of a squid giant axon. 
Time marks on the right in msec. 



in the upper two records is often called a ' negative' 
after-potential, because an action potential was con- 
sidered in the classical physiology as a " negative' 
variation of the potential of the nerve surface (cf. p. 
105). Evidently, the term 'negative' after-potential is 
at present confusing and inadequate. 

The after-potential of the frog (or toad) muscle 
fiber (fig. 34, right top) seems to decay roughly at 
the time constant of the membrane (30). This after- 
potential is not associated with any measurable change 
in the membrane resistance. These facts suggest that, 
following one whole cycle of activity of the muscle 
fiber membrane, there is an excessive charge of elec- 
tricity remaining in the large capacity of the mem- 
brane and this charge is dissipated through the mem- 
brane resistance. In the nodal membrane of the toad 
(or frog) nerve fiber, the time constant of the mem- 
brane is far shorter than the duration of the spike 
potential (table i, p. 89); therefore, an after-poten- 
tial of this type does not exist in the amphibian nerve 
fiber. 

The after-potential of the frog nerve fiber shown in 
figure 34, right center, was induced by poisoning the 
fiber with veratrine, an alkaloid which is known to 
cause rhythmical activity in the mu.scle and nerve. 
Gasser & Graham (39) have shown that this chemical 
greatly enhances the (negative) after-potential of the 
nerve trunk. The after-potential of this type is asso- 
ciated with a concomitant decrease in the membrane 
resistance (108, 133). 

The after-potential in the squid giant axon i.fig. 34, 
right bottom) is often referred to as an 'undershoot': 
the membrane potential stays, after the end of the 
main spike potential, below the initial level of the 
resting potential. As we have seen in the record of 
figure 12, this after-potential is associated with a 
pronounced decrease in the membrane resistance. 
Grundfest el al. (45) found that there is a phase of 
slightly increased membrane impedance following the 
period of decreased membrane impedance. In the 
sodium theory (p. 118), the undershoot in the squid 
giant axon is attributed to an increase in the potas- 
sium permeability of the membrane. 

The nature of the after-potentials in B- and C- 
fibers in the vertebrate nerve is not clear. Further 
discussions on the after-potentials of the nerve trunk 
are found in the monograph bv Gasser & Erlanger 
(38). 

Rhythmical Activitj! 

In excitable tissues in living organisms, action po- 
tentials appear, as a rule, in more-or-less rapid sue- 



lib 



HANDBOOK OF PHVSIOLOGV 



NEUROPHVSIOLOCi' I 



cession. Thus motor nerve cells in the vertebrate 
spinal cord discharge impulses repetitively over a 
wide rang;e of frequency depending; on the state of 
the cell. Similarly, sensory nerve fibers carry a series 
of impulses toward the spinal cord in response to 
sensory stimuli delivered to their endings. There is 
at present a large amount of data concerning the 
pattern of impulse discharge obtained by the method 
of recording single fiber responses originated by 
Adrian (3, 4). 

In many excitable tissues, application of a long 
constant current generates a train of action potentials, 
as shown by Arvanitaki (8), Fessard (33), Erlanger & 
Blair (29), Katz (72) and others. The records fur- 
nished in figure 35 show repetitive firing of action 
potentials in the .squid giant a.xon induced by con- 
stant outward membrane currents of four different 
intensities. The stimulating pulses are sent into the 
a.xon through a long intracellular metal wire elec- 
trode, and the responses are recorded with another 
intracellular electrode. It is difficult to maintain 
repetitive firing indefinitely under these experimental 
conditions. It is to be observed that each action po- 
tential is preceded by a slowly rising phase of the 
membrane potential. This slowly rising phase has 
been demonstrated at the sites of naturally induced 
repetitive responses in the automatically beating 
cardiac muscle [cf. VVeidmann (143)]. 

The site at which impulses are initiated repetitively 
is called a 'pacemaker'. At present, it is not clear how- 
sensory nerve endings or the motor nerve cells become 
pacemakers. However, there is one thing that can be 
inferred from the mechanism of the nervous conduc- 
tion in the peripheral nerve fiber. As has been dis- 
cussed on previous pages, nervous conduction is ef- 
fected through excitation of each segjment (or node 



of RanvierJ l)y the electric current generated b\ the 
adjacent active segment. From this one can infer that 
a sensory stimulus or a natural stimulus for the motor 
nerve cell has to be transformed eventually into an 
electric stimulus in order that it initiates a propagated 
impulse. (If the size and shape of the electric current 
generated by a sensory stimulus are similar to those 
of the ordinary action current, the statement just 
made has no meaning; however, it is generally ac- 
cepted that the first electrical sign of the response to 
a sensory stimulus is variable in size and very differ- 
ent from the ordinary all-or-none response.) Since a 
constant current applied to a peripheral nerve fiber 
can gi\'e rise to a repetitive firing of impulses, it is 
generally believed that natural pacemakers resemble 
in some respect an artificial one induced i)y applica- 
tion of a constant current (fig. 35). 

The mechanism of repetitive firing proposed by 
Adrian (3, 4) to interpret the injury and sensory dis- 
charges of impulses is as follows. An electric stimulus 
of a constant intensity sets up the first action potential 
in accordance with the law of electric excitation. 
Then, the nerve fiber falls into the refractory period 
which makes the stimulus totally ineffective. As the 
fiber recovers from this refractoriness, the stimulus 
becomes effective again and the second action po- 
tential is set up. The second response leaves behind 
it another refractory period. The nerve fiber thus 
exhibits a kind of oscillatory phenomenon similar to 
that in a neon lamp connected to a battery, a con- 
denser and a resistor. 

It is simple to express Adrian's concept in terms of 
the membrane potential and the threshold depolariza- 
tion. At the beginning of the refractory period, the 
critical membrane potential is close to the level of 
the shoulder of the action potential (see fig. 20). 




FIG. 35. Repetitive firing of action potentials in a squid giant axon. The relative intensities of the 
stimulating currents used are indicated by the broken lines. Both stimulating and recording elec- 
trodes were long intracellular metal wires. [From S. Hagiwara e! al., unpublished.] 



CONDUCTIOiN OF THE NERVE IMPULSE 



11/ 



During the relatively refractory period there is a con- 
tinuous recovery in the threshold membrane potential. 
This concept of Adrian seems to explain many facts 
known al:)out repetitive firing. In tissues with a time 
constant which is much longer than the duration of 
the action potential, however, not only the recovery 
process, but also the time required to charge the 
membrane capacity is considered to influence the 
rhythm of repetitive firing (54). It is also known that 
the oscillation in the membrane potential at sub- 
threshold levels (8, 9) plays an important role in 
production of rhythmical activity in some tissues. 

In connection with the pacemaker mechanism, 
there is an interesting phenomenon which seems to 
deserve a short discussion. That is 'resetting' of the 
rhythm of the repetitive response by an ' extra im- 
pulse' reaching the pacemaker. In 1936 Gilson (41) 
examined the effect of an artificial (electric) stimula- 
tion of the sinus of the turtle heart upon the rhythm 
of the heart beat. He found that the time interval 
between the artificiallv induced response and the 
following (natural) response is approximately equal 
to the normal inter\'al of the automatically induced 
responses, regardless of the interval between the 
artificially induced respon.se and the preceding one. 
Similar phenomena have been demonstrated in 
natural and artificial pacemakers in the sensory 
nerve fiber and in the motor nerxe fiber [cf. Tasaki 
(121)]. 



CURRENT THEORIES OF THE RESTING .-^ND 
.ACTION POTENTI.ALS 

In the last section of this chapter, we shall briefly 
discuss the current theories dealing with the mecha- 
nism whereby the resting and action potential of the 
nerve or muscle fiber is generated. This problem has 
been extensively and authoritatively reviewed by 
many recent inv estigators in a svmposium Electrochemis- 
try in Biology and Medicine, edited by Shedlovsk\- (l 1 1). 
The great variety of the views maintained by recent 
investigators toward the present problem indicates 
that the current theories to be described below are 
not yet accepted as unequivocal. We shall make an 
attempt to explore the sources of equivocalities and 
controversies in the present problem. 



Resting Potential 

Twenty years before the turn of the century, 
Biedermann (12, p. 354) discovered that application 



of an isosmotic potassium chloride solution to a por- 
tion of a muscle generates a large potential difference 
between the site of application and the remaining 
surface of the muscle. Later, Hober (49) extended 
this observation and found that the ability of various 
cations to affect the resting potential of the muscle 
increases in the following series: Li, Na, Mg, Cs, 
NH4, Rb, K. Hober found also that the correspond- 
ing series for anions is CNS, NO3, I, Br, CI, acetate, 
HPO4, SO 4, tartarate. 

In 1902 Bernstein (10) published the .so-called 
' membrane theory' in which he postulated a) that 
the resting potential is pre-existent at the plasma 
membrane of the cell (prior to injury or application 
oi potassium salts), and h) that the resting potential 
is maintained by virtue of the semipermealsility of 
the plasma meinbrane. At that time, the pre-existence 
of ions in the electrolyte solution (Arrhenius, 1883) 
was known, and osmotic phenomena in the mem- 
brane of some plant cells and in artificial membranes 
(Pfeffer, 1877) were also well understood. Nernst's 
book on theoretical chemistry dealing with concentra- 
tion cells had just appeared at that time (1900). 

A present, there is no doubt about the validity of 
the membrane theory in the form described above. 
There are in Bernstein's theory two additional postu- 
lates. He speculated that the resting potential is a diffu- 
sion potential resulting from the difference in the mo- 
bility of potassium and phosphate ions through the 
membrane and also that the action potential is caused 
by a reduction of the resting potential resulting from 
a nonspecific increa.se of permeability of the mem- 
brane during activity. 

Later on, a large volume of work was published 
showing that, within a certain limit, the relationship 
between the resting potential, Er, and the external 
potassium concentration, [K]o, can be expressed by 
the Nernst equation 



E, = 58 los r— r ^'"^'^ 



(.2-0 



where [K]i represents the concentration of potas- 
sium in the protoplasm (7, 55, 68, 76, 94). However, 
the validity of equation (12-1) does not by itself 
prove that the process of diffusion of potassium ions 
is responsible for the resting potential. 

Equation (12-1) represents the theoretical maxi- 
mum (absolute) value of the resting potential that 
can be attained if the concentration gradient of 
potassium were the cause of the resting membrane 
potential. If, therefore, it happens under any circum- 



HANDBOOK OF PHYSIOLOGV 



NEUROPHYSIOLOGY I 



Stances that the observed membrane potential ex- 
ceeds the value given by equation (12-1), one is 
forced to believe that the resting potential is gener- 
ated primarily by some electrochemical mechanism 
other than the diffusion of the potassium ion. This 
type of evidence against the potassium theory has 
been expressed by several in\estis;ators though not 
in a written form until the recent work of Shaw et al. 

(109)- 

The electrochemical nature of the plasma mem- 
brane is not yet clearly understood. Osterhout (94), 
Beutner (ii) and others assume that the resting 
potential is maintained across an oil (nonaqueous) 
layer. Teorell (139), SoUner (112) and others have 
developed the concept of a charged porous mem- 
brane as the site of bioelectric potential. .Shedlo\'skv 
(iio) stressed the asymmetry of the membrane with 
respect to two surfaces and the possible role of protons 
in generation of the bioelectric potentials. 

To explain the divergence of the obser\ed resting 
potential from the Nernst equation, Hodgkin (55) 
used the modified Goldman equation (42). There is 
.some doubt as to the applicability of this equation 
to li\ing cells, because of the assumption of a uniform 
field (i.e. no charge in the membrane) adopted in 
deriving this equation (139, p. 338). Boyle & Conway 
(17) found that the ratio of chloride across the muscle 
fiber membrane is close to the ratio [K]o/[K], and 
argued that the resting potential of the skeletal 
muscle fiber is a Donnan potential. There are, how- 
ever, some arguments against this notion (44). 

Actum Poleiilial 

There is at present only one widelv accepted 
theory of action potential production. That is the 
so-called sodium theory postulated by Hodgkin & 
Huxley (57, 58, 59). Previously Nachmansohn 
(89) advanced a theory in which acetylcholine is 
assumed to play a decisive role in action potential 
production. Recently, however, he shifted his effort 
toward an attempt to supply a biochemical basis 
for the sodium theory (90). 

This theory started with the de\elopmcnt of the 
modern technique of recording and controlling the 
intracellular potential. When it was found that the 
amplitude of the membrane action potential is sub- 
stantially larger than the resting potential across the 
memjjrane (p. 84), physiologists realized that Bern- 
stein's postulate as to the origin of the action potential 
(p. 117) is incorrect. The finding of Hodgkin & 
Katz (62^ that the amplitude of the action potential 



of the squid giant axon varies with approximately 
58 mv times the logarithm of the concentration of 
sodium in the external medium (p. 93) has led 
the.se British physiologists to postulate that the mem- 
brane potential at the peak of acti\its- is determined 
by the concentration gradient of the .sodium ion 
across the axon membrane. (According to this pos- 
tulate, the amplitude of the action potential should 
vary with 58 m\- times the logarithm of the intracel- 
lular concentration of sodium; however, it is difficult 
in practice to alter the sodium concentration in a 
wide range.) 

Hodgkin & Huxley (59) elaborated this concept 
further and explained the mechanism of action po- 
tential production by assuming that the increase in 
the membrane conductance during activity (p. 89) 
is a specific increase of permeaijility to sodium ions. 
They tried to substantiate this idea by voltage clamp 
experiments (p. 91). Their success in reconstruct- 
ing the action potential from the data obtained by 
the voltage clamp technique is often regarded as 
sufficient proof of the sodium theory. 

The diatjram in figure 36, right, shows the equiva- 
lent circuit of the excitable membrane postulated in 
the theory. When the membrane is at rest, the con- 
ductance of the membrane is maintained by the per- 
meabilit\' of the membrane to potassium ions; i.e. 
gK » gsii! where gk is the 'potassium conductance' 
and g^a the 'sodium conductance' of the membrane. 
This situation should iiring the potential of the 
resting membrane close to E^i which is defined by 
equation (12-1). E^ia in the diagram represents the 
'sodium equilibrium potential' defined ijy the equa- 
tion of the type of equation (12-1) for the sodium 
ion; the polarity of E^., is opposite to that of Ek- If 
g^:, increases at the peak of activity to a \alue well 
aboN'e gK, the niemljrane potential should approach 



RESTING 
+- + -»- 4- 




RESTING 
+ -1- + + + + 


+ + +- 


_ 




ACTIVE 
REGION 


-1- 


+ + + *-^ 




FIG. 36. Right. ■ The equivalent circuit proposed by Hodgkin 
& Hu.xley to represent tiie membrane of the squid giant 
axon. Left: The state of an axon carrying an impulse proposed 
by the same authors. The signs 4- and — indicate the electric 
charges on the capacity which are assumed to determine the 
membrane potential. Note that this concept of charges on the 
condenser determining the membrane potential is inapplicable 
to the circuit diagram of fig. 9C. 



CONDUCTION OF THE NERVE IMPULSE 



"9 



E^,,; this explains the reversal of the membrane poten- 
tial during activity. If ^^a is increased to some extent 
b\ a stimulating current pulse, a further increase in 
gif., can be brought about by a regenerati\e process; 
an increase in ^^a causes a rise in the memijrane 
potential which in turn gives rise to a further increase 
in gf^i,. The theory is self-consistent. The readers who 
are interested in this beautiful scheme are referred to 
the original article (59). 

It may be worth pointing out that there are in the 
sodium theory a number of assumptions that are not 
directly proved by experiments. They assume in the 
first place that the axon membrane under voltage 
clamp is spatially uniform; this may not be a safe 
assumption. They assume also that the capacit\- of 
the membrane is connected in parallel to the cmf of 
the membrane (p. 85). They did not exclude the 
possibility that the sodium ions bound in the sub- 
stance of the membrane (instead of the free sodium 
ions in the medium) exert direct influence upon the 
amplitude of the action potential. There are several 
more assumptions in the theory. Although most of 
these assumptions appear to be reasonable, it is also 
true that one can make a set of entirely difTerent 
assumptions and explain almost the same amount of 
experimental data. 

There is at present a large volume of work dealing 
with the movement of sodium or potassium ions 
across the excitable membrane. The principal findings 



pertinent to the discussion in this chapter are a) a 
steady outward current through the axon membrane 
is carried almost exclusively by potassium ions (60), 
and b) there is an exchange of intracellular potassium 
with extracellular sodium associated with repetitive 
excitation of the axon (74). It is generally agreed that 
the amount of the Na-K exchange associated with 
repetitive excitation observed in invertebrate axons 
is close to the \alue expected from the sodium theory. 
It should Ije kept in mind in this connection that 
there are excitable tissues which do not require any 
sodium ion in the medium to produce action potentials. 
Crustacean muscles studied by Fatt & Katz (31) 
are a well-known example, and the plant cell, Nilella, 
investigated by Osterhout and his associate (93, 94) 
is another. This fact suggests that the role of the 
sodium ion in the medium inight be only indirectly 
connected with the process of action potential pro- 
duction. The alternatis'e explanation of this fact is 
that the mechanism of action potential production is 
verv different in difTerent tissues. 



The author wishes to express his gratitude to the following 
colleagues who have kindly read the manuscript of this chapter 
and have given many important suggestions; Dr. M. Fuortes, 
Dr. S. Hagiwara, Prof. A. L. Hodgkin and Dr. C. S. Spyro- 
poulos. The manuscript was prepared with the valuable help 
of Mrs. Mary Allen, Mrs. Claire Mayer and Mrs. Lydia N. 
Tasaki, to whom the author also wants to express his apprecia- 
tion. 



REFERENCES 



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1931/32- 



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NEUROPHYSIOLOGY 



28. 
29- 

30- 
3'- 

33- 

34- 

35- 

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CHAPTER IV 



Initiation of impulses at receptors 



J. A. B. GRAY I Department oj Physiology, University College, London, England 



CHAPTER CONTENTS 

General Properties 

Type of Energy Required to Excite Receptors 

Adaptation 

Receptive Fields 

Information 
Repetitive Responses and Tonic Receptors 

Stimulus-Frequency Relations 

The Effect of a Reduction of Excitation 

The Nature of Repetitive Firing 

Modification of Afferent Discharges by Current 
Excitation of Impulses by Controlled Pulses and Phasic Re- 
ceptors 

Quantitative Aspects of Excitation 

On and Off Responses 

Summation 
Receptor Potentials and Other Generator Potentials 

Generator Potentials in Complex Organs 

Receptor Potentials Generated in Nerve Terminals 

Relation of Receptor Potentials to Impulse Initiation 

Quantitative Relations between Stimulus and Receptor 
Potential 

Absolute Magnitude of the Receptor Potential 

Summation of Receptor Potentials 

Depression 
Site of Impulse Initiation 

Effect of Procaine and Sodium Lack on Receptor Potentials 
Transmission of Energy to the Receptor Elements 
Effects of 'Transmitter' Substances 

Action of Acetylcholine 

Action of Blocking Agents and Anticholinesterases 

Effects of Sympathetic Stimulation and Epinephrine 

Other Substances 
Minute Structure of Receptors 
Hypotheses Concerning the Mechanisms of Receptors 



IN THE INTACT ORGANISM impulses are set up in pri- 
mary afferent fibers as a result of activity in those 
receptors with which the fibers are associated. These 
receptors may consist solely of specialized termina- 
tions of the afferent nerve fibers, or the nerve endings 



may be associated with other cells which play a 
significant role in the initiation of impulses. In either 
instance, the role of the receptor is to record the 
state of, or changes in, the physical or chemical en- 
vironment by the initiation of impulses which are 
then conducted in the primary afferent fibers to the 
central nervous system. A primary afferent fiber may 
be connected with a single receptor or with many; 
but even when it is supplied by numerous receptors, 
a single afferent fiber remains a single channel into 
the central nervous system and must be considered as 
such. When dealing with the activity in such a fiber 
it is necessary to consider the fiber and all its periph- 
eral connections as a whole, that is as a sen.sory unit. 
It is the purpose of this section to consider something 
of the general behas'ior of sensory units and of the 
mechanisms by which indi\idual receptors initiate 
impulses in the primary afferent neurons. 



GENERAL PROPERTIES 

A few words should first be said concerning classi- 
fication. Sensory units can be described by reference 
to the properties of the specific stimulus, the nature 
of the activity and the site and distribution of the re- 
ceptive field. All these factors, together with the con- 
duction velocity of the fiber, are measurable quanti- 
ties and a precise description of a sensory unit is thus 
possible. It seems better in this context to avoid terms 
such as warmth, pain or red; these terms describe 
sensations which depend on the activity of the whole 
nervous system, not just on the properties of one 
sensory unit. 

Type of Energy Required to Excite Receptors 

In most animal organisms there are receptors that 
respond to the following forms of energy: mechanical, 



123 



[24 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



thermal, electromagnetic (as light) and chemical. 
With the exception of one important group, it ap- 
pears that nearly all receptors are especially sensitive 
to one form of energy and either completely or rela- 
tively insensitive to all others. The excepted group 
consists of those receptors that have a low sensitivity 
to all types of energy, but will respond to any form 
of energy that reaches a damaging or near damaging 
level; these receptors are of course those that give 
rise to defensive reflexes and are associated with the 
sensation of pain. The specificity of receptors to a 
particular form of energy was first propounded in 
modern times by Miiller (76). As a whole this con- 
cept is not seriously challenged, but recently an 
attack has been made on its application to receptors 
situated in the skin (99). The objections are based 
both on the finding that there are areas of human 
skin in which morphologically specialized endings 
are not found but from which all modalities of sen- 
sation can be elicited (39), and on the results of 
certain sensation experiments (65, 66). There need 
be no rigid correlation between morphological and 
functional specializations; it is indeed interesting to 
note that the bulk of the direct evidence for the func- 
tional specificity of sensory units has come from 
preparations of frog and toad skin which contain 
few morphologically differentiated nerve endings 
(24). As regards to sen.sation experiments it must be 
realized that sensations are the end result of compli- 
cated processes and that such experiments, while 
giving information about sensations and their specific- 
ity, cannot weigh heavily against direct evidence on 
the properties of sensory units. 

Direct evidence of the specificity of units has been 
obtained by recording the responses of single ones to 
different stimuli. In the earlier experiments of this 
type single fibers were not isolated in an anatomical 
sense, but small bundles of nerve fibers were used so 
that the activity of indi\idual units could he identi- 
fied and analyzed; using this technique it was possible 
to show that therrnal and near-damaging stimuli 
only excited activity in small fibers and did not 
produce activity in the larger fibers which responded 
only to mechanical stimuli (2, 50, 103, 104). This 
type of work has now been carried a stage further by 
isolating and recording from single afferent fibers 
that have their receptive fields in the skin of toads and 
of cats (73). Results of such experiments show that 
mechanically sensitive units are not easily excited by 
thermal stimuli [though this has been shown to hap- 
pen in the cat's tongue (45)] or by acid; that ther- 
mally sensitive units do not normally respond to 



mechanical stimulation [the rattlesnake pit organ is 
an exception (15)]; and that units responding to 
acid, prick or burning do not respond to small 
mechanical or thermal stimuli. 

The specificity of sensory units is not confined, 
however, to a simple distinction between different 
types of energy but involves distinctions between 
other properties of the stimulus. Of these properties, 
those connected with its time course are perhaps the 
most obvious; the different rates of adaptation ex- 
hibited by different units is an example which will 
be considered again. .Specificity to a particular band 
of frequencies of a periodic function is another ex- 
ample; thus there is evidence that different units in 
the retina respond to different frequencies of light 
waves (31, 91) and that primary units from the 
mammalian cochlea have particular characteristics 
in relation to the frequency of the sound waves (93). 
In both these instances the sensory units are display- 
ing a specificity, ijut there is clearly a difference in 
the way this specificity is brought about. In the 
retina it seems probable that individual receptors are 
different, but in the cochlea it is the mechanical 
properties of the system that are mainly responsible 
for the results. Such a distinction between the proper- 
ties of the receptor and the properties of the support- 
ing tissues is one that ari.ses in other situations but is 
one that is irrelevant in the context of describing the 
properties of sensory units. The examples of specificity 
so far given in this paragraph are concerned with 
time factors, but there are others. Thus there are 
two types of thermal unit found in the cat's tongue; 
in both types the frequency of the impulse discharge 
depends on the temperature of the receptors, but in 
one group the maximum frequency is found at a 
temperature of 30 to 32 °C (46), while in the other it 
occurs at 37.5 to 4o°C (22). Again, units in the cat's 
tongue responding to chemical stimuli, and pre- 
sumably responsible for the sensation of taste, can 
be grouped in respect to the substances that are able 
to set up activity in them (82). 

Adaptation 

When a piece of tissue containing a receptor sensi- 
tive to mechanical stimuli is subjected to an abrupt 
increase in the forces applied to it and the new situa- 
tion is then maintained, the sensory unit will dis- 
charge impulses at a frequency, which starts at a 
relatively high value and then decreases with time 
(4, 74, 75) (fig. i). This decline in frequency is known 
as adaptation and may be slow or rapid. In those 



INITIATION OF IMPULSES AT RECEPTORS 



12- 



units that are described as tonic, the frequency of 
the impulse discharge declines relatively slowly to a 
steady value which is characteristic of the applied 
force (fig. i); the frequency of the discharge from 
other units, those called phasic, adapts more rapidly 
and finally falls to zero (5). In an extreme case a 
sensory unit may only discharge a single impulse 
during the change in the applied forces and will 
then remain silent until another change takes place. 
Adaptation is also observed in sensory units specifi- 
cally sensitive to forms of energy other than mechani- 
cal. 

Adaptation is a word that describes the response of 
a sensory unit to a particular function of the type of 
energy concerned. When adaptation is rapid, it can 
be said that the unit is not signalling a pressure, a 
temperature or a concentration; but it does not tell 
us what particular function in respect to time is 
signalled. 

To say that a function is signalled means that a 
constant value of the function gives rise to a constant 
and repeatable frequency of impulse discharge in the 
fiber of the sensory unit. In most situations it is very 
difficult to maintain a constant velocity or accelera- 
tion for a sufficiently long time to see whether or not 
a constant frequency of discharge is in fact set up 
[a notable exception has appeared in the experiments 
on the semicircular canals using constant angular 
velocities and accelerations (72)]. Even if such ex- 
periments were performed it is by no means certain 
that simple relations would be found. This is there- 



fore a situation in which it is necessary to continue to 
use an empirical description. 

It should be noted that in the first sentence of this 
section, reference is made to the tissue surrounding 
the receptors. Even in the instances in which a recep- 
tor has been isolated, e.g. the muscle spindle and 
Pacinian corpuscle, there is far more supporting tis- 
sue than active element. These supporting tissues 
may be of fundamental importance in the adapta- 
tion of 'simple' receptors in the same way as the 
structures of the middle ear and cochlea cause 
'adaptation' of the ear to steady pressures applied 
to the tympanic membrane. This problem will be 
considered at a later stage when all the relevant 
evidence has been discussed. 

Receptive Fields 

A sensory unit has a particular situation and par- 
ticular size of receptive field, i.e. the area from which 
the single afferent fiber receives branches. The size 
of these receptive fields can vary quite considerably, 
for example up to 9 by 5 cm, not mm, in cat's skin 
(73) and up to 100 sq. mm in frog's skin (3); while 
other sensory units have receptive fields which com- 
prise only a single end organ. Variation of size of re- 
ceptive fields occurs with different types of unit in 
skin and also in specialized organs such as the eye. 
There is wide overlap of receptive fields and it is 
clear that spatial discrimination must depend on the 
coordination of information supplied through a con- 
sideraljle number of primary channels. 




FIG. I. Response of cat muscle spindle to stretch. Abscissa: 
time in sec. Ordinate: impulses frequency per sec. Each curve 
for a diflferent force. [From Matthews (75).] 



Information 

Sensory units constitute independent channels 
which signal to the central nervous system informa- 
tion about the physical and chemical environment of 
the organism. This information is conveyed by the 
pattern of activity in any one unit and by the charac- 
teristics and organization of each channel. These 
factors can be classified as follows: 
a) Factors related to time 

i) Interval between impulses 
ii) Duration of activity 
h) Factors related to the properties of units 

i) Characteristics of the 'normal' stimulus, 
e.g. the nature of the energy and other 
relevant factors 
ii) Size and position of the receptive field 
iii) SensitivitN' of the unit 



i->6 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



REPETITIVE RESPONSES AND TONIG RECEPTORS 

Stimulus-Frequency Relations 

There are many sensory units, the function of 
which is to signal to the central nervous system the 
properties of a steady state, e.g. temperature, con- 
centration or intensity of illumination. At any time, 
except shortly after an aljrupt change from one state 
to another, the frequency of the impulse discharge 
of the unit will depend on the value of the physical 
or chemical function in question; and a particular 
frequency will, in the working range of any one unit, 
be consistently related to a particular value. One 
example is seen in figure 2 where the frequency of 
impulse discharge in five single fibers from pressure 
receptors of the carotid sinus is plotted against pres- 
sure in the sinus. Another example appears in figm-e 
22 of Chapter XVIII on Thermal Sensations in this 
volume (p. 452), in which the impulse frequencies in 
two units from the cat's tongue responding to thermal 
stimuli are plotted against temperature. The curves 
in the two figures are clearly quite different; the 
pressure units, while showing individual variations, 
all start to fire at a certain pressure above which the 
frequency of discharge increa.ses as the pressure in- 
creases until an upper limit of frequency is reached 
(12, 60). The temperature units on the other hand 
both show maxima in their temperature-frequency 
relationship, but these maxima occur at two widely 
different temperatures. The two types of response 
represent the activity of two distinct groups of units 
found in cats (22, 46) and it is presumed that the 




FIG. 2. Responses of five single pressure sensitive units QA 
to £) from the cat's carotid sinus. Abscissa: intrasinusal pres- 
sure in mm Hg. Ordinate: impulse frequency per sec. [From 
Landgren (60).] 



activity of these two types of unit bears a close causal 
relationship with the subjective sensations of cold 
and warmth. 

Looking at the two examples shown, it would seem 
improbable that any relationship between 'stimulus' 
and frequency having general relevance to sensory 
units of all types could be found. This is strictly true, 
but there is a relationship that has been found to de- 
scribe reasonably well the response characteristics of 
certain types of unit in their working range. This is 
what is known as the Weber-Fechner law. This law 
derives from an observation made by Weber that the 
smallest difference in the weight of two objects bears 
a constant relation to the weight of the objects. It is 
usually given as AI/I = C, where / is intensity of 
stimulus. A/ the smallest detectable difference in 
intensity and C a constant. Fechner developed this 
observation in a theoretical way by making the as- 
sumption that each discriminable step of stimulus 
intensity corresponds to a imit increase in sensation, 
that is to say he stated that AI/I = kAS where AS 
is the increase in sensation. From this it follows that 
d5/d/ = I /k/ and S = a log / + A. This equation 
was originally put forward in an attempt to quanti- 
tate .sensation, a thing we are not concerned with 
here; howe\er, we are concerned with its relevance 
to 'stimulus' -frequency relations. The relation be- 
tween the applied force and impulse frequency re- 
corded from a frog's muscle spindle has been found 
to be consistent with this relationship (95). The cor- 
responding relationship between intensity of illumina- 
tion and response from an ommatidium of the eye of 
Limiilus is also consistent with the equation under 
certain specific conditions (41). These findings have 
inevitably raised the question of whether this rela- 
tionship indicates anything about the mechanisms 
invoked in the initiation of impulses or whether it 
must be regarded simply as an empirical description 
(31). The fit between equation and experiment is not 
sufficiently good to suggest that the fundamental 
processes depend on a simple logarithmic relation- 
ship, but if, as seems possible, the.se processes are 
related to ionic equilibria across cell membranes, a 
logarithmic term might he expected to appear in the 
relationship. 

Effect fij a Reduction oj Excitation 

It has already been pointed out that while a tonic 
sensory unit will respond to a certain steady state 
with a certain frequency, a sudden increase in, for 



INITIATION OF IMPULSES AT RECEPTORS 



127 



instance, the applied force will cause a relatively large 
increase in the frequency of the discharge, an increase 
which will then decline until the correct frequency 
for the new steady state has been reached. A similar 
process occurs if there is a sudden decrease in, again 
for instance, the applied force. In this instance the 
frequency falls abruptly to a value below that ex- 
pected for the new steady state and then increases 
with time. Thus, if a muscle spindle is discharging 
rhythmically and the muscle in which it lies is 
stretched for a time and then suddenly returned to 
its resting length, the frequency of the discharge from 
the spindle falls well below its resting value, possibly 
to zero; after a time the resting rhythm re-establishes 
itself (75). Similar changes can be observed in other 
types of unit, for example in temperature sensitive 
units (46), and the pressure sensiti\e units of the cat's 
carotid sinus (60). It should be noted that the beha- 
vior of such units contrasts with that of phasic units 
which are considered in another section below. 

Nature of Repetitive Firing 

Ideas on the mechanisms by which firing takes 
place started with the proposals of Adrian (i). Essen- 
tially these were that special nonaccommodating 
regions of nerve exist at sensory nerve endings and 
that repetitive activity is initiated in these regions; the 
frequency of the discharge depends on the refractory 
period which may be longer here than in other parts 
of the nerve. Broadly speaking, work on the nature of 
repetitive firing by sensory receptors has followed two 
lines. The first has attacked the problem of nerve 
accommodation and the other, the mechanism that 
determines the interval between impulses. 

Many investigations have been carried out on the 
rate of accommodation of nerve and these have shown 
that accommodation need not be rapid and that in 
crustacean (49), amphibian (26) and mammalian 
(32, 89) nerve it is in fact possible to obtain main- 
tained repetitive firing during the passage of a con- 
stant current. Further it has been found that most 
experimental procedures tend to increase the rate of 
accommodation (81); it is possible that the common 
eflfect of all these procedures is to lower the membrane 
potential, a reduction of which is known to increase 
the rate of accommodation (94). These findings led 
to the view that the mechanism of repetitive firing 
from sensory receptors could be explained on the 
known properties of nerve fibers. This view was 
elaborated in particular by certain Scandinavian 



workers (9, 30) who suggested that the receptor 
develops a 'generator potential' which causes current 
to flow in the nerve fiber so acting like a constant 
current stimulus in setting up a train of impulses. 
This idea has remained the basis of most subsequent 
work on the subject. 

The concept that the inter\als between the im- 
pulses of a train are dependent on the rate of recovery 
after an impulse is faced with the difficulty that 
rhythmic discharges of very low frequency, a few 
impulses per second, can be obser\ed. These intervals 
are much longer than the total duration of the re- 
covery process as known in nerve. Investigations on 
the repetitive firing of crustacean nerve during the 
passage of a constant current have introduced another 
idea C49)> that the intervals between impulses are 
determined by the response time. That is to say the 
intervals are determined in the same manner as the 
latency from the l:)eginning of a current stimulus to 
the initiation of the first impulse. 

The passage of a constant current through a crusta- 
cean axon sets up a repetitive discharge as shown in 
figure 3. Several points can be seen in this figure; the 
frequency of discharge is related to the current 
strength; the interval between the beginning of the 
current and the first impulse is always closely related 
to the intervals between the other impulses; these 
intervals are all dependent on the development of 
the local response, an impulse being initiated when- 
ever this local response reaches the critical potential; 
the critical potential at which the impulses are set up 
is the same with all but the greatest strengths of cur- 
rent and all but the highest frequencies of impulses. 
Apart from this direct evidence that it is the time 
course of the development of the local response that 
sets the interval between impulses, the recovery time 
of these axons is such that it cannot explain the fre- 
quencies observed. These crustacean axons have long 
response times and can therefore give regular low 
frequency discharges. 

The events taking place in certain stretch receptors 
in Crustacea are very similar (27). A microelectrode 
in the cell body of one of these primary sensory neu- 
rons is able to detect a receptor potential generated in 
the terminals and, superimposed on it, a discharge of 
nerve impulses. The receptor potential will be con- 
sidered in a later section. Here it is sufficient to point 
out that after an impulse the membrane potential 
builds up again in a manner very similar to that 
shown in figure 3, and the next impulse is set up 
when this potential reaches the critical value. The 



121: 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 




50 50 

AAA/WWWW\ AA/\/WWWW\=^^/"" 



eye./ 



FIG. 3. Responses at the cathode of single carcinus a,\ons to 
constant currents. A-H: increasing currents as indicated. /-A .■ 
near threshold currents at higher amplification and faster sweep 
speed. L. potential change at anode, conditions as /-A. [From 
Hodgkin (49).] 



critical potential remains the same at all but the 
highest values of impulse frequenc\ . 

The initiation of impulses b\ the receptor potential 
generated in the muscle spindle of the frog has also 
been observed (58). In this preparation the critical 
potential remains constant for all except the first im- 
pulse in a discharge at a constant frequency, but the 
value is different for different frequencies; in fact 
there is a direct and linear relationship between the 
value of the critical potential and the frequency of 
the discharge. An explanation of this phenomenon 
has been given as follows (58) : recovery after a nerve 
impulse depends on two processes o) a restoration of 
membrane resistance and 6) a return of excitability 
(see 48). If the first of these processes is the more 
rapid in the frog muscle spindle fibers, but not in 
the crustacean fiber, then results such as have been 
observed would be expected. 



Modifuation 0/ Afferent Discharges by Current 

Afferent discharges can be modified by the applica- 
tion of currents to the regions in which such dis- 
charges are set up. This can be seen in the frog's 
muscle spindle (25); if the spindle is made to dis- 
charge at a suitable frequency by stretch and a current 
is applied between an electrode on the afferent nerve 
and another on the muscle, the frequency is increased 
if the electrode on the muscle is the cathode and de- 
creased if this electrode is the anode. The increase or 
decrease of frequency is related to the intensity of 
current, though the relation is not a simple one. Other 
preparations exhibit similar effects. Current passed 
through the nerve terminals of the isolated labyrinth 
of the rav causes an increase in the frequency ot 
discharge in these fibers when the cathode is on the 
tissue surrounding the .sensory endings and the anode 
on the afferent nerve fibers; a current in the opposite 
direction causes a reduction in the discharge fre- 
quency (71). These changes caused by the flow of 
current summate with those due to angular accelera- 
tion in the appropriate direction. Similar results can 
be observed by polarization of the lateralis organs of 
Xenopus laevis. It has been shown that when the applied 
current flows along the nerve fiber, as in the instances 
already described, an increase in frequency occurs 
when the cathode is on the terminal and the anode on 
the nerve; however, if the current flows between elec- 
trodes placed on either side of the skin, the frequency 
is increased when the cathode is on the inside and the 
anode on the outside (77). The frequency of discharge 
in the nerve fibers from the lateral line organ of the 
Japanese eel is also increased by a current passed 
between an anode on the outside of the skin and a 
cathode on the inside (55); the passage of a current in 
the same direction has been shown to excite afferent 
fibers from touch receptors in frog skin (73). Currents 
can al.so modifv the discharge from a compound eye 
(40). 

These results are important in two respects. First, 
depolarization of the terminal parts of the axon 
membrane can summate with end organ activity 
which suggests that the latter involves a depolariza- 
tion of the terminals. This is in fact known to occur 
in manv instances which will be considered below. 
Second, it can be argued from the results obtained 
with currents pas.sed across the skin instead of along 
the ner\e that, during sensory activity, impulses are 
initiated away from the terminal (77)- Direct evi- 
dence that this is so in certain instances will be given 
later. 



INITIATION OF IMPULSES AT RECEPTORS 



129 



EXCITATION OF IMPULSES BY CONTROLLED PULSES 
AND PHASIC RECEPTORS 

In the last section stimulus-frequency relations 
were considered. Such relations give important in- 
formation about units that signal the values of steady 
states by indicating them as particular and repeatable 
frequencies of impulses. That is to say these relations 
are important for nonadapting or tonic units. On the 
other hand, the response of phasic units, and the 
adapting part of responses of tonic units, are de- 
pendent on the time course of the stimulus; in partic- 
ular the rates of change at the beginning and end of 
the pulse are important. To investigate these phasic 
units in detail, it is therefore important to use stimuli 
of known time course. It is also important that the 
stimulus should be adequately damped. The im- 
portance of this can be shown by an example: Pacin- 
ian corpuscles have thresholds of a few tenths of a 
micron and, for the amplitude threshold to be mini- 
mal, the displacement must be complete in less than 
a millisecond (34); if large displacements of tens of 
microns are used, it only requires a one per cent oscil- 
lation to give rise to what appears to be a repetitive 
response. Various techniques have been used for this 
purpose. Thus for mechanical receptors, electro- 
magnetic (6, 57) and crystal transducers (34, 35) 
have been used. The former have bigger displace- 
ments, but generally have a slower time course than 
the latter which can have a damped rise time of 0.2 
msec, and a displacement of 10 to 20 /j. It should be 
noted that even 0.2 msec, is not very short compared 
with the latency from the beginning of the stimulus 
to the impulse. 

Quantitative Aspects of Excitation 

Using such methods, the latencies for impulse ini- 
tiation in Pacinian corpuscles and frog skin receptors 
have been measured (34, 35). In the Paciniaii cor- 
puscle latencies after the onset of mechanical deforma- 
tions of any duration are longer (i.e. 0.5 to 3.0 msec.) 
than those following the beginning of a constant 
current stimulus to the receptor's own nerve fiber 
within a millimeter of the ending. After mechanical 
stimulation of frog skin even longer latencies have 
been observed. The latency observed in the Pacinian 
corpuscle can be shown to be due to the time taken 
for the receptor potential to develop (37); it seems 
likely, therefore, that the longer latencies found with 
frog skin receptors indicate even more prolonged 
receptor proces.ses. Curves of recovery after the ini- 



tiation of an impulse by a short mechanical pulse to 
a Pacinian corpuscle have been shown to be similar 
to the curves of recovery obtained after electrical 
excitation of the ending's own nerve fiber close to the 
corpuscle and of nerves in general (34). Thus, in this 
instance at least, there is direct evidence that the 
time course of recovery at the site of impulse initiation 
is not much different from that in other parts of 
nerves. 

The change of amplitude threshold with change of 
stimulus velocity has also been measured, and the 
minimum velocity of stimulus necessary for excitation 
found. Thus, just as there is a critical slope in the 
excitation of nerve by a linearly increasing current, 
so there is a critical slope in the excitation of phasic 
receptors by linearly increasing displacements. Such 
measurements give a quantitative measure of the 
adaptation of such receptors. Thus the critical slope 
for a Pacinian corpuscle is given as 1 200 rheobases 
per sec. (36) and that for receptors in frog's skin 61 
rheobases per sec (35). 

As a means of investigating the fundamental mech- 
anisms of receptors, such measurements have been 
superseded by direct recording of receptor potentials; 
but they are still of use in certain types of quantitative 
investigation (53). 

On and Off Responses 

At least some phasic receptors respond with one or 
a few impulses to a change from one state to another; 
this response is not qualitatively dependent on the 
sign of this change. Thus many photoreceptors re- 
spond when the intensity of illumination on them is 
suddenly raised from one level to another and again 
when the intensity is suddenly reduced (30). The same 
type of response to change of state is seen in receptors 
in toad and cat skin (73). Measurements of the 
threshold amplitude for on and oft" responses to rec- 
tangular displacements have been made for Pacinian 
corpuscles and frog skin receptors; in the former the 
threshold for a compression (the 'on response') is 
usually slightly lower than that for a decompression 
(the 'off response'), but not infrequently the reverse is 
true (34); on the other hand the excitability of the 
frog's cutaneous receptors to a compression is much 
greater than the excitability to the decompression 
(35). These difTerences may well be due to the 
mechanics of the systems, for in these experiments 
compression is a result of an externally applied force, 
while decompression depends solely on the restoring 
forces inherent in the tissue; it is likely that restoration 



I30 



HANDBOOK OF PHYSIOLOGY 



NELROPHYSIOLOGY I 



is a much more rapid process in a Pacinian corpuscle 
than in frog's skin. 

Summatiim 

Two subthreshold short pulses applied to a phasic 
receptor within a suitable interval of each other can 
summate and set up an impulse; the essential point 
in this experiment is that the first pulse is over before 
the beginning of the second and the summation takes 
place in the receptor. Further discussion of this point 
will be left to the ne.xt section where receptor poten- 
tials are discussed. One particular case can, however, 
be discussed here. It is possible to observe summation 
between the subthreshold activity evoked by a small 
short mechanical pulse and a brief electrical test 
shock. .Such a test shock can be used to measure the 
excitability of the receptor at different times after the 
application of the mechanical pulse; in this way in- 
direct evidence of the time course of a receptor poten- 
tial has been obtained (34)- 



RECEPTOR POTENTIALS AND OTHER 
GENERATOR POTENTIALS 

It is now widely held that the immediate cause of 
impulse initiation in receptors and sense organs is the 
development of an electrical potential change which is 
graded according to certain characteristics of the 
stimulus and which is confined to the region of the 
receptor or organ. .Such potentials have now been 
found in a number of situations of different types and 
these findings, together with supporting evidence such 
as summation results from other sites, form the justi- 
fication for such a generalization. 

In this .section I shall use the term 'generator po- 
tential' to describe any graded potential change oc- 
curring in a sensory receptor or in a complex sense 
organ that can reasonably be supposed to be a cause 
of the initiation of an impulse. The term 'receptor 
potentials' I will confine to those generator potentials 
occurring in a single receptor. Thus the cochlea 
microphonic is a generator potential but not a re- 
ceptor potential. 

Generator Potentials in Complex Organs 

These lie outside the scope of this particular chapter 
but are included briefly for completeness. The cochlea 
is the best example of this group. In this organ there is 
a potential difference maintained between the endo- 



lymph and the perilymph (96). During the application 
of a sound wave, an alternating potential can be re- 
corded and shown to have its greatest intensity at the 
point on the basilar membrane at which the hair cells 
are situated (16). This potential is directly related to 
the sound pressure wave (100). There is reason to 
suppose that this microphonic potential, as it is called, 
is the cause of impulse initiation (16). 'Microphonic' 
potentials have also been found in other sites, e.g. 
the lateral line organs (54) and sacculus (105). These 
potentials serve a similar function to the receptor 
potentials of neurons but, in the cochlea at least, 
they represent changes of potential between multi- 
cellular compartments instead of across cell mem- 
branes. It is not improbable that there are common 
factors in the development of these two types of po- 
tential, but we cannot expect to find close parallels. 

Receptor Potentials Generated in Nerve Terminals 

.Such potentials have been recorded from certain 
mechanically excitable receptors (6, 27, 37, 58}, from 
photoreceptors (42, 79) and from olfactory receptors 
(78). In all these instances the receptor potential has 
been recorded at a distance from its source, and in no 
case has the membrane potential of the receptor 
region been recorded directly. In each of the three 
mechanical examples on which we have information 
at present, the records were obtained by recording 
the currents flowing along the nerve fiber, the nerve 
fiber behaving as a pair of passive concentric conduc- 
tors. The changes in these currents must have been 
related to changes in potential across the membrane 
of the terminal portions of the afferent nerve fiber, 
since all currents recorded must have crossed the 
membrane peripheral to the recording region; this 
does not prove of course that the changes are actively 
generated across the terminal membrane. Reasons for 
believing that the receptor potentials are in fact 
actively generated at this site are given in the last 
section of this chapter. 

Examples of receptor potentials are shown in 
figure 4. Figure 4.-1 and B are records from muscle 
spindles from the frog (58); in both experiments the 
preparations had been procainized to prevent impulse 
activity. Figure ^A shows the changes that occur at 
the beginning of a maintained stretch; it can be seen 
that there is a relatively large initial change of poten- 
tial and that this is foOowed by a small but main- 
tained potential change. The earlier phase, called the 
dynamic phase, is related to the velocity of the 
stretch. The smaller maintained change of potential 



INITIATION OF IMPULSES AT RECEPTORS 



'S' 




FIG. 4. Receptor potentials from different receptors. .1 and B. from frogs muscle spindle, pro- 
cainized. Top: stretch. Bottom: receptor potential. Time, A, 500 cps; B, o.i sec. [From Katz (58).] 
C: from cat's Pacinian corpuscle, procainized. Upper trace {at starty amplitude and duration of 
displacement and time in msec. Note that this trace crosses the other trace during displacement. 
Lower trace (at starty receptor potential record. [From Gray & Sato (37).] D: from crayfish 
stretch receptor. Arrows mark duration of stretch. Time, i sec. [From Eyzaguirre & Kuffler (27).] 



depends only on the amplitude of the stretch. Figure 
4B shows also the events occurring when the stretch is 
released. It can be seen that there is a change of 
potential in the opposite direction to the other deflec- 
tions, that is the electrode near the receptor goes posi- 
ti\e to the distant electrode. The tiine course of this 
deflection tends to be slower than that of the initial 
dynamic phase, but it must be remembered that 
relaxation of the muscle depends on the restoring 
forces in the tissue while the stretch is actively im- 
posed. The three phases of the receptor potential 
correspond to the initial burst, to the maintained 
discharge and to the reduced discharge that follows 
the end of a stretch. 

Figure 4C (37) shows a receptor potential from a 
Pacinian corpuscle and with it the voltage pulse ap- 
plied to the crystal transducer that was used to stimu- 
late; impulse activity has been prevented with pro- 
caine. This potential differs in several respects from 
that found in the muscle spindle. There is no main- 
tained plateau, the potential declining to zero once 
the peak is past. The shape of the receptor potential 
is nearly or completely the same whether excited by 
a short pulse of say 0.3 msec, duration, Ijy the be- 



ginning of a long pulse (fig. 4C) or by the end of a 
long pulse. As in the case of the muscle spindle these 
results are consistent with the results of experiments 
on the excitation of impulses by these receptors. These 
two examples illustrate contrasting types of receptor 
potential, the one associated with tonic behavior and 
the other with phasic behavior. In particular it is 
worth noting that a receptor potential, and with it an 
impulse, is set up by decompression of a Pacinian cor- 
puscle, while relaxation of a muscle spindle is asso- 
ciated with a positive going receptor potential and 
an inhibition of the impulse discharge. 

The receptor potential in figure 4Z) is that of a 
slowly adapting stretch receptor from the crayfish 
(27). This was recorded by means of a microelectrode 
in the cell body of the neuron which, in this instance, 
lies in the periphery close to the muscle; the receptor 
part of the cell lies still further to the periphery in the 
terminations that ramify in the receptor muscle. The 
record shows a steady depolarization well maintained 
throughout the stretch; there is no marked dynamic 
phase, even when the early part of the potential is not 
obscured by spikes, as in figure 4Z), though there is 
soine initial decline in the level of the depolarization; 



'32 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



at the end of the stretch the potential simply returns 
to its resting level. This difference from the muscle 
spindle potentials may be due to the effective velocity 
of the stretch. In this record, unlike those that are 
illustrated in figure ^A, B and C, the impulse dis- 
charge has not been interfered with and five spikes 
are shown arising from the receptor potential. It can 
be seen that each impulse is preceded by a relatively 
slow decrease in membrane potential (the prepoten- 
tial) and that when this decrease reaches a critical 
value the impulse is discharged. 

Receptor potentials are not confined to mechani- 
cally excited receptors. It has long been known that 
slow potentials could be obtained from the retina and 
from compound eyes (31). There has been reason to 
suppose that part at least of these potentials repre- 
sented activity of the receptors themselves. Direct 
evidence that single ommatidia produce receptor 
potentials has now been obtained. In the single 
isolated ommatidium of Limulus (42) a receptor 
potential builds up rapidly when the ommatidium 
is illuminated. The potential then dies away, but 
with suitable recording conditions some depolari- 
zation appears to remain as long as the receptor is 
illuminated. The cessation of illumination is not 
accompanied by hyperpolarization. The olfactory 
mucosa of the frog produces slow potential changes 
when excited by air containing a suitable agent (78). 
The distribution in area and in depth of these 
potentials and their relative insensitivity to cocaine 
suggest that they are due to synchronous activity of 
the olfactory receptors. 

Relation of Receptor Potentials to Impulse Initiation 

There can be little doubt that receptor potentials 
are the immediate cause of the initiation of impulses. 
They always precede the impulse and the impulses 
appear when a critical potential has been reached. 
In the crustacean stretch receptor this critical level 
remains constant under a variety of conditions. With 
the frog's muscle spindle the critical potential depends 
on the frequency of the discharge, an observation 
which has been discussed above. In this preparation 
the frequency of discharge is linearly related to the 
amplitude of the receptor potential, a fact which sug- 
gests that the receptor potential is causally related to 
the impulse discharge. That such a relationship is not 
immediately visible in the results obtained from the 
crustacean stretch receptor does not mean, of course, 
that the frequency of the impulse discharge is not 
related to the amplitude of the receptor potential. 



This is perhaps best explained by considering the 
steps involved in the initiation of the impulse. There 
are reasons, which will be considered below, for sup- 
posing that the impulses are initiated at a point which 
is near but not identical with that at which the re- 
ceptor potential is generated. Currents due to the 
receptor potential will then flow through and dis- 
charge the membrane of the neighboring parts of the 
nerve fiber; this part of the membrane will develop 
local responses (48, 56), and if the membrane poten- 
tial falls to the critical level an impulse will he 
discharged. This sequence of events is essentially the 
same as that found during the repetitive firing of a 
carcinus axon in response to an externally applied 
constant current (49). The slowly rising prepotentials 
of the crayfish stretch receptor (fig. 4Z)) are similar 
to those of the current excited carcinus axon (fig. 3). 
In both these examples the recording conditions are 
such that what is recorded is related to the membrane 
potential at the point of impulse initiation and not 
to the intensity of the charging current or the po- 
tential of the source supplying this current. In conse- 
quence what is seen is the passive discharging and 
local response of the membrane at the site of initiation 
followed by the impulse if and when the memlDrane 
potential falls to a critical level; this part of the 
membrane is then repolarized and the cycle starts 
again. The rate of discharging of the membrane and 
hence the frequency of the impulses depends on the 
intensity of the discharging current which in turn 
depends on the size of the receptor potential; this, 
however, is masked during a train of impulses. If the 
amplitude of the receptor potential could be measured 
during the impulse discharge a relationship between 
receptor potential amplitude and frequency would 
no doubt be found, and this might be similar to the 
relation Ijetween applied current and frequency in 
the carcinus axon. A relation was found in the case of 
the frog's muscle spindle because, between impulses, 
conditions were such that the full amplitude of the 
receptor potential was recorded; this was proved by 
subsequent procainization. Possible reasons for this 
behavior have already been considered. 

Qjiantitatwe Relations Between Stimulus and 
Receptor Potential 

The amplitudes of the receptor potentials of the 
muscle spindle and Pacinian corpuscle increase with 
the amplitude of the displacement up to a certain 
point and then level off to a maximum. An example is 
shown in figure 5. This particular example was ob- 



INITIATION OF IMPULSES AT RECEPTORS 



'33 



100 r 
% 
90 



70 
60 
SO 

■10 
)0 
20 - 
10 



F19.5 



Stiinulus scrength 



10 
1 



Fig 6 



o O 



oo 



10 
2 















0° 






Stimului strength 






IS 20 


25 


30 


3 4 


S 


« 



FIG. 5. Receptor potential amplitude in relation to the displacement of the mechanical stimulus 
with velocity constant in a Pacinian corpuscle. Abscissa: stimulus strength in arbitrary units. Ordi- 
nate: receptor potential amplitude as percentage of maximum. O same points as •, but stimulus 
strength scale expanded five times. [From Gray & Sato (37).^ 

FIG. 6. Receptor potential rate of rise in relation to the displacement of the mechanical stimulus 
with velocity constant in a Pacinian corpuscle. Abscissa: stimulus strength in arbitrary units. Ordi- 
nate: receptor potential rate of rise as percentages of maximum amplitude per msec. O same points 
as •, but stimulus strength scale expanded five times. [From Gray & Sato (37).] 



tained from a Pacinian corpuscle (37), but a siinilar 
relationship has been observed in the muscle spindle 
of the frog (58}. The value of the receptor potential 
reaches a constant level with large displacements; this 
is not conclusively proved by the published data, 
partly because of the limits to the size of stimulus used 
and partly, in the Pacinian corpuscle experiments, 
because the biggest stimuli introduced artifacts that 
tended to sum with the response. The question of the 
absolute amplitude of this ma.ximum is considered in 
a later section. 

The rate of rise of the potential is also related 
to the size of the exciting displacement (37). This is 
shown in figure 6. There is a change in the slope of 
this graph at that level of stimulus strength above 
which the amplitude increase is limited, but even 
above this point the rate of rise of the potential con- 
tinues to increase with stimulus strength. That this is 
a genuine effect is supported h\ the fact that the time 
of rise of the potential continues to shorten over this 
range of stimuli. Since the recorded potential is a 
result of a potential change across the terminal mem- 
brane (whether or not the potential is actively gen- 
erated at this site), the rate of rise of the receptor 
potential will reflect the rate at which current flows 
in to the capacity of this membrane. In other words 
these results suggest that the current across the mem- 
brane of the nerve fiber terminal continues to increase 



as the stimulus increases even though the peak po- 
tential has reached a maximum \alue. 

The amplitude of certain receptor potentials, for 
example those of the Pacinian corpuscle and the 
early phase of that of the frog's muscle spindle (fig. 
\A), is also dependent on the velocity of the displace- 
ment. Indirect evidence shows that this is also true of 
other receptors responding to other forms of energy, 
for example thermal receptors (46). Figure 7 illus- 
trates the change in relati\e ainplitude of the receptor 
potential that accompanies change in the velocity of 
the mechanical stimulus, the amplitude of the stimu- 
lus ijeing kept constant. It is immediately clear that 
the amplitude of the receptor potential, while inde- 
pendent of velocity at high values, is over a certain 
range closely related to the velocity of the stimulus. 
The 'angle' of this curve occurs, for the Pacinian 
corpuscle, at a compression velocity of about i mm 
per sec. (i.e. about 5 thresholds per msec). This 
means that many physiological stimuli may be ex- 
pected to lie within the velocity-sensitive range. 

The time course of the receptor potential is in most 
instances dependent on the properties of the stimulus. 
It has already been pointed out that the rate of rise 
of the potential \aries with stimulus amplitude and 
this change in the rate of rise of the potential change 
is accompanied by a change in the time of rise (37, 
58). The rate of rise of the potential change may also 
be affected by the \elocity (or comparable time 



'34 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



100 

% 

K 
00 
70 
M 
SO 
40 
M 
20 
10 




•• • 



Stimulus velocity (V/miec) 
_I I 



FIG. J. Receptor potential amplituide in relation to the 
velocity of the mechanical stimulus with displacement constant 
in a Pacinian corpuscle. Abscissa/ stimulus velocity in arbitrary 
units. Ordinate: receptor potential amplitutie as percentage of 
maximum. [From Gray & Sato (37).] 



function of the relevant form of energy) of the pulse 
used to excite; thus the rate of rise of the receptor 
potential of the frog's muscle spindle gets less as the 
velocity of the stimulus is reduced. In the Pacinian 
corpuscle there may Ije some change in rate of rise, 
but often there is no effect attributable specifically to 
the stimulus velocity; that is to say that, though the 
rate of rise of the potential change increases as its 
amplitude increases, the time course of a receptor 
potential of a given amplitude is often the same 
whether it is produced by a small displacement 
having a high velocity or by a larger displacement of 
lower velocity. In other words there are many me- 
chanical pulses having different values of amplitude 
and velocity that are equivalent as 'stimuli'. 

The duration of static receptor potentials, e.g. that 
of the frog's muscle spindle and the slowly adapting 
stretch receptor of the crayfish, is directly dependent 
on the duration of the applied force. The rate of decay 
of those potentials, which are velocity sensitive, may 
possibly depend on the duration of the applied force 
under certain circumstances; however, the rate of 
decay of the receptor potentials of the Pacinian cor- 
puscle, the only end organ in which this particular 
point has been investigated, is normally independent 
of the duration of the stimulus (37). Off responses 
have the same time course as on responses. 

Absolute Magnitude oj the Receptor Potential 

Receptor potentials reach a ma.ximum at a certain 
value of stimulus strength. It is of considerable theo- 



retical importance to know the absolute value of this 
potential change. Up to the present, it has been pos- 
sible to make only a rough estimate of its value in the 
Pacinian corpuscle (20). This has been done by re- 
cording the external current flowing along the axon 
between the second and third nodes of Ranvier during 
activity of each of these nodes and of the receptor 
potential. By the use of blocking techniques and by 
taking diff"erences, the.se components were obtained 
separately and measured. Under suitable conditions 
these currents will he proportional to the driving 
potentials. The results given are that the receptor 
potential amplitude is 59 per cent (n = 6, S.D. = 
14 per cent) of the amplitude of the impulse at node 2 
and 38 per cent (n = 5, S.D. = 17 per cent) of the 
amplitude of the impulse at node 3. The difference 
between the figures is due to a decline in the impulse 
amplitude as the terminal is approached. The atten- 
uation per internode of the receptor potential is likely 
to be less than the 0.5 for large myelinated fibers of 
toads C92), so the absolute value of the receptor 
potential can be considered as of the same order of 
magnitude as the resting and action potentials. This 
conclusion is supported by results from the crayfish 
stretch receptor (27). The amplitude of the recorded 
receptor potential at threshold ranges from 8 to 25 mv 
depending on the type of receptor. It has been esti- 
mated that the loss due to passive conduction along 
the nerve filler will have reduced the true value of the 
receptor potential by 20 to 80 per cent; also a maxi- 
mum receptor potential must be appreciably greater 
than a threshold one. The ratio for the Pacinian 
corpuscle is 10 to i (37). 



Summation oj Receptor Potentials 

If, during a maintained receptor potential, the re- 
ceptor is subjected to increase in the stimulus strength 
the final value of the receptor potential will correspond 
to the final value of the stimulus. In this instance both 
the stimulus and the receptor potential have summed. 
With short pulse excitation it has been shown that 
summation of receptor potentials occurs after the 
stimulus is over (6, 37) as shown in figure 8. This 
summation appears similar to that found with end- 
plate potentials and synaptic potentials. A special case 
of summation occurs when an 'on' response summates 
with an 'ofli" response (37). Summation of sub- 
threshold receptor potentials can in this way set up 
impulses (6) and it seems likely that this process is of 



INITIATION OF IMPULSES AT RECEPTORS 



135 




FIG. 8. Summation of receptor potentials with different 
intervals between stimuli. Upper trace: stimulus signal and time 
in msec. Lower trace: receptor potentials. [From Gray & Sato 
(37)-] 



considerable functional importance in determining 
maximum sensitivities. It is probably also important 
in determining the thresholds for sensation at different 
frequencies of vibration (83). 

Depression 

After the production of one receptor potential by a 
Pacinian corpuscle a subsequent one, occurring within 
a few milliseconds, is depressed. This is most easily 
seen with a preparation in which impulse activity has 
been prevented; it can then be seen that the depres- 
sion of the test responses increases as the conditioning 
stimulus is increased and decreases as the interval be- 
tween the conditioning and test pulses is increased 
(18, 37). Depression of the receptor potential is also 
caused by an impulse set up as a result of mechanical 
stimulation; this depression is much greater than that 
produced by a threshold receptor potential alone, 
though it does not appear to be as great as the de- 
pression caused by really large mechanical stimuli, 
whether an impulse is present or not. Antidromically 
conducted impulses also cause depression of a subse- 
quent receptor potential, though for any given time 
interval after the impulse the depression is slightly less 



than when the conditioning impulse is excited me- 
chanically. At the time of writing there are a number 
of problems which require elucidation and on which 
the e\idence is conflicting. 

Depression has not been described for other recep- 
tor potentials, but this is not surprising as the stim- 
ulating conditions have been very different. It would 
be interesting to know, however, if any part of the 
initial decline of other receptor potentials were due 
to the same cause as this depression; the decline of 
the Pacinian corpuscle potential appears to be due 
to other and more rapid processes. 

SITE OF IMPULSE INITIATION 

There is evidence from the rapidly adapting type 
of stretch receptor of the crayfish that impulses are 
set up in the cell body (27). The records in these ex- 
periments were made through an electrode that was 
inside the cell body and it was found that the change 
of memijrane potential required to excite an impulse 
was the same whether this change was brought about 
by a receptor potential spreading from the periphery 
or by current spread from an antidromically con- 
ducted impulse that had been blocked before it 
invaded the cell body. If the receptor potential set up 
impulses peripheral to the cell body, the apparent 
threshold value of the receptor potential, as recorded 
by this method, would be less than the true value by 
the amount of decrement occurring between the site 
of initiation and the cell ijody; this is in fact what 
occurs in the slowly adapting stretch receptors of the 
same species. Direct distortion of the cell body and 
the larger dendrites does not produce any potential 
changes; receptor potentials are produced only by 
stretching the muscle fibers in which the finer termi- 
nals of the neuron ramify. It therefore seems certain 
that while the impulses are initiated in the cell body 
of the rapidly adapting receptor, the receptor poten- 
tials are developed peripheral to this in the finer 
dendritic terminals. 

A similar state of affairs appears to occur in the 
Pacinian corpuscle (20). In this receptor a straight 
nonmyelinated fiber of 2 pi diameter runs down the 
central core of the corupscle; at the end of this central 
core the axon becomes myelinated. One node of 
Ranvier is regularly found inside the corpuscle about 
half way between the end of the central core and the 
point at which the axon leaves the capsule, and the 
second occurs near the latter point (86). The imme- 
diate surroundings of the nonmyelinated terminal 



.36 



HANDBOOK OF PHYSIOL(K;V 



NEUROPHYSIOLOGY I 



have been shown to be specialized (85), and it must 
be supposed that it is in this region that the receptor 
potential is generated. By recording across a barrier 
surrounding the internode between the second and 
third nodes of Ranvier, it was found possible to record 
distinct phases of activity due to each of the first two 
nodes if the thresholds of these nodes were raised by 
anodal polarization. No phase of impulse activity 
could be found attributalile to the nonmyelinated 
terminal even though thresholds were raised by an 
amount that, on theoretical grounds, should have 
been quite adequate to reveal such impulse activity 
if it existed. It therefore appears that after a mechani- 
cal stimulus the impulse is set up at the first node of 
Ranvier. 

Indirect evidence that impulses are not initiated in 
the terminations of the afferent nerve fibers of certain 
other preparations has already been considered in the 
section on the effects of applied currents. 

Not only is there evidence that impulses are, in 
some receptors at least, set up away from the termi- 
nals in which the receptor potentials are generated, 
but there is also evidence that such terminals are not 
invaded by antidromically conducted impulses. In 
the crayfish stretch receptor the receptor potential 
is not abolished by an antidromic action potential; if 
the impulse invaded the membrane that is involved 
in the production of the receptor potential one would 
expect a complete short circuiting of this membrane 
and the temporary abolition of the receptor potential 
(28). A similar observation has been made with the 
olfactory mucous membrane of the frog (78); stimula- 
tion of the olfactory nerve at different strengths and 
frequencies had no effect on the response of the olfac- 
torv membrane to an exciting .substance. As has 
already been stated in the last section, an antidromic 
impulse causes slightly less depression of the receptor 
potential in the Pacinian corpuscle than does an 
impulse set up by a mechanical pulse. It has already 
been argued that an impulse initiated in this receptor 
by a mechanical stimulus starts at the first node of 
Ranvier; if an antidromically conducted impulse in- 
vaded the nonmyelinated terminal then it would be 
expected to produce a greater depression of the 
receptor potential. This is not the case and it seems, 
therefore, that antidromic impulses do not invade the 
nonmyelinated terminal (18). 

Evidence that impulses set up by physiological 
stimuli to receptors do not start in the receptor region 
might simply mean that all-or-nothing impulses can- 
not occur there during receptor activity. That anti- 
dromic impulses do not invade the terminals might be 



a result of Ijlock at regions of low safety factor, though 
from parallel situations elsewhere this does not seem 
ver) likely. The most probable explanation of all 
these results is that those regions of membrane that 
are not in\aded are different from the rest of the 
neuron surface and are not capable of producing a 
regenerative all-or-nothing response. 

When a frog's muscle spindle is discharging at low 
frequency small all-or-nothing potentials can be seen. 
These are much smaller than the propagated impulse 
and may occur in a number of discrete sizes (57). 
They disappear if the frequency of discharge of full- 
size impulses is increased and also if the receptor is 
bombarded antidromically. After a full-size impulse 
there is always a delay before the next all-or-nothing 
event, whether full-size or small, but after one of the 
small all-or-nothing potentials the interval may be 
quite short. An explanation of these events may be 
that impulses are set up in the terminal branches of 
this type of receptor, but an impulse in a single branch 
is unable to pass the regions of low safety factor that 
occur where the branches join (57). A full-size im- 
pulse would then only be set up if there were sufficient 
synchrony in the activity of the terminal branches. 
On the same argument all-or-nothing activity in a 
single branch would fail to invade other branches and 
therefore would not depress their activity, while a 
full-size antidromic impulse would invade them all. 



EFFECT OF PROC.MNE .J^ND SODIUM LACK ON 
RECEPTOR POTENTIALS 

In the frog's muscle spindle concentrations of pro- 
caine from 0.1 to 0.3 per cent abolish impulse acti\ity 
but leave the receptor potential apparently unaffected. 
Higher concentrations of procaine reduce the ampli- 
tude of the receptor potential, affecting the static 
phase more than the dynamic (58). Similar results 
can be obtained with the Pacinian corpuscle of the 
cat. The impulse is abolished by concentrations of 
o.i to 0.5 per cent procaine in the bathing fluid, but 
if the procaine is washed out after aijout 10 min. 
there is no reduction in the amplitude of the receptor 
potential. Prolonged soaking in these concentrations 
causes a reduction of the receptor potential amplitude 

(37)- 

Similar eff"ects can be obtained in both these prep- 
arations if they are soaked in sodium-free solutions. 
Ten minutes .soaking in such a solution abolishes 
repetitive firing from the muscle spindle while thirty 
minutes is enough to abolish the initial spike (58). 



INITIATION OF IMPULSES AT RECEPTORS 



137 



Thirty minutes soaking is about the time needed 
to abolish the impulse from a Pacinian corpuscle 
preparation (37). In neither instance is the receptor 
potential effected. 

The times of action of these solutions are remark- 
ably similar for the two preparations and in both 
instances are very long compared with the time such 
solutions take to act on isolated single nerve fibers. It 
seems likely that diffusion times play an important 
part. It is known from direct experiments with 
labelled sodium, potassium and bromine that diffu- 
sion through the capsules of the Pacinian corpuscle is 
slow C38). 

In the Pacinian corpuscle, however, it is possible to 
perfuse the receptor through the capillary loop that 
enters the corpuscle with the axon and ramifies in its 
proximal pole (19). Using a perfused preparation of 
this kind it is found that procaine in a concentration 
of 0.02 to 0.05 per cent in the perfusion fluid abolishes 
the impulse; 0.05 per cent and higher concentrations 
of procaine cause a reduction of the receptor potential 
amplitude. The abolition of the impulse occurs within 
1.5 min. 

If these preparations of the Pacinian corpuscle are 
perfused with a sodium-free solution the amplitude of 
the receptor potential falls and after about 20 min. 
the amplitude is constant and very small. This is 
illustrated in figure 9. This reduction in amplitude 
occurs whether the sodium chloride of the physi- 
ological solution is replaced by choline chloride or 
by sucrose. The effect, under faNoraisle conditions, 
is reversible and recovery occurs on changing the 
perfusion fluid back to a physiological solution. 
When different concentrations of sodium are per- 
fused it is found that the amplitude of the receptor 
potential, measured after a constant le\el has been 
reached, is related in a graded manner to the con- 
centration of sodium. When sodium is absent there 
is a small remnant of the receptor potential; it is 
probable that this represents a genuine property 
of the receptor (19). 

The receptor potentials of other types of receptor 
have also been found to be resistant to local anes- 
thetics. Cocaine (0.5 per cent) applied externally has 
little or no effect on the potentials of the olfactory 
mucous memljrane, though the same application 
abolishes the responses of the olfactory bulb (78). 
Procaine in concentrations of 0.05 to o. i per cent in 
the bathing fluid abolishes the impulses but not the 
receptor potential of the crayfish stretch receptor. 

The position at the present time seems to be that 
while receptor potentials are more resistant than 




FIG. 9. Effect of perfusion with a sodium-free solution on 
receptor potential amplitude. Abscissa: time in min. Ordninle: 
receptor potential amplitude, arbitrary units. Sodium chloride 
was replaced with sucrose and changes in recording resistance 
have been corrected for. Impulses were abolished with pro- 
caine but were allowed to reappear during the period marked 
by the dotted line. [From Diamond, J., J. A. B. Gray & D. 
Inman. Unpublished figure.] 



impulses to procaine, in the Pacinian corpuscle at least 
quite low concentrations (0.05 per cent) do affect 
the receptor potential if the diffusion barriers are 
avoided by perfusion. Perfusion also reveals that the 
receptor potential is almost completely abolished in 
the absence of sodium. 



TRANSMISSION OF ENERGY TO THE RECEPTOR ELEMENTS 

It has long been recognized that there are factors 
in the transmission of the exciting energy to the re- 
ceptors that are important in the functioning of the 



138 



HANDBOOK OF I'HVSIOLOGV 



NEUROPHYSIOLOGY I 



more specialized sense organs. For example the 
ability of the cochlea of the higher vertebrates to act 
as a frequency analyzer is due to its mechanical prop- 
erties (97). In compound eyes the distribution of 
absorbing pigments affects the distribution of light on 
the receptors so as to increase cither the sensitivity 
or the discrimination of the eye (84). The same situa- 
tion can be seen if the skin is taken as a whole. It has 
been shown that thermal receptors respond to the 
temperature at a given point at a given time (106); 
the distribution, both in time and space, of tempera- 
ture in the skin, and consccjuently the nature of 
sensation aroused, will depend on the physical proper- 
ties of the whole system. Another, and rather different, 
example of the effect of external physical factors is 
the decrease in the rate of adaptation of mechanical 
receptors in frog's skin that occurs as a result of 
stretching the skin (68). 

All the examples mentioned in the last paragraph 
refer to the physical properties of a whole tissue or 
organ and their effect on the behavior of a population 
of receptors. The factors involved in the transmission 



of energy inside what is normally described as a 
single ending can also be of fundamental importance. 
The Pacinian corpuscle consists of a central core sur- 
rounded by thin laminae which form the boundaries 
of coaxial spheroids; the spaces between the laminae 
are filled with fluid. When the ending is .squeezed 
displacements of the laminae occur and these can be 
recorded from photographs taken with short flashes 
(51, 52). During and immediately after the onset of a 
compression, relatively large displacements of the 
laminae occur (fig. 10 lejt); but these decline rapidly 
to a steady value which is maintained as long as the 
corpuscle is compressed. This maintained displace- 
ment \aries with the position of the lamina measured, 
those near the periphery of the corpuscle showing large 
displacements while those near the center show none; 
figure 1 1 is a plot of maintained displacement against 
distance from the center of the corpuscle. The time 
course of the compression can be recorded and there- 
fore the displacement that would be expected at any 
instant, if the response of the system were inde- 
pendent of time, can be calculated. Subtraction o 





2 4 




70 



so 



Fig. II 




.III 



flOO 



II 200 



2>L 



300 



500 



600 M 700 



FIG. 10. Mechanical properties of the Pacinian corpuscle. Left: time course of displacements 
of 3 laminae (see inset) during a compression that started at / = o, rose linearly to / = 2.6 msec, 
and then remained constant. Right: dynamic component' of displacement. See text. [By courtesy 
of S.J. Hubbard.] 

FIG. II. Mechanical properties of the Pacinian corpuscle. Abscissa: diameter in the transverse 
plane (2r). Ordinate: maintained displacement of laminae as functions of transverse diameters 
(■2Ar). t marks edge of the central core. Bars indicate ±2 X standard error. [By courtesy of S 
J. Hubbard.] 



INITIATION OF IMPULSES AT RECEPTORS 



'39 



this theoretical displacement from that observed 
(fig. lo left) leaves a 'dynamic component' (fig. 
10 right); it can be seen that this component is 
transmitted with less attenuation to the center of the 
end organ, and also that its time course is similar to 
that of a receptor potential (fig. 4C). It seems there- 
fore that the rapid adaptation of this receptor is 
primarily a mechanical phenomenon. Since neither a 
change in axon length nor a bending of the axon has 
been detected, it seems that radial displacements of 
the axon itself, or of the tissues immediately sur- 
rounding it, are responsible for activating the re- 
ceptor. 



EFFECTS OF TR.'SiNSMITTER SUBSTANCES 

A number of investigations into the actions of 
acetylcholine, epinephrine, histamine and related 
compounds have been carried out. These investiga- 
tions have in general had one of two objecti\es: one, 
to see if these substances are normally involved in the 
initiation of impulses by receptors; the other, to see 
if there is specialization of the membrane of the ter- 
minal part of the sensory axon. 

Action of Acetylcholine 

Acetylcholine has been shown to increase or initiate 
a discharge of impulses from a variety of sensory 
receptors. These include mechanical receptors from 
the skin of the cat and the dog (13, 23), from the cat's 
carotid sinus (17, 64), from the crayfish stretch re- 
ceptor (102), the cat's tongue (62) and from the 
frog's skin (53); also thermal receptors in the cat's 
tongue (21) and chemical receptors of the cat's 
tongue (62) and carotid body (98). Succinylcholine 
has been found to increase the activity of mammalian 
muscle spindles (33). Finally acetylcholine has been 
found to effect and even initiate sensations in the 
human subject; these include pain (7, 44, 90) and 
thermal (10) sensations. Many of these investigations 
include control experiments designed to show that 
these are direct effects on the sensory pathway and 
are not secondary to contractions of smooth or striated 
muscle and do not result from excitation of the auto- 
nomic nervous system. It seems clear therefore that 
acetylcholine does have an action on some part of the 
sensory pathway, and since similar applications of 
acetylcholine to nerve fibers (53, 70) or to pregangli- 
onic nerve terminals (11, 14) are ineffective, it seems 
likely that these results represent a direct action of 



the substance on the receptor mechanism itself. The 
dosage and pharmacological pattern of these re- 
sponses vary from one preparation to another. The 
most common picture is that represented by the ex- 
periments on the mechanical receptors of cats and 
frogs in which responses were recorded directly from 
the primary sensory nerve fibers. These responses are 
produced by doses of the same order of magnitude as 
those required to excite the skeletal neuromuscular 
junction. They are unaffected by atropine, but are 
blocked by curare or excess nicotine; smaller doses 
of nicotine beha\e like acetylcholine. The picture is 
thus very similar to that of the acetylcholine action at 
synapses and the skeletal neuromuscular junction. 
The main divergence from this pattern is that atropine 
blocks the acetylcholine effect in the crayfish stretch 
receptor (102). Atropine has also been found to raise 
the thresholds for the sensations of pain (90) and of 
cold (lo) in the human; its mode of action in these 
instances is not at present clear. 

There has Ijeen some difference of opinion as to 
whether acetylcholine can act independently or 
whether it merely sensitizes the receptor to the 
natural stimulus; it is possible that the action may be 
different in different preparations. In some prepara- 
tions, as shown in figure 12, there is no doubt that 
acetylcholine can initiate a discharge (17) and that 
the action of acetylcholine summates with the physio- 
logical stimulus (17, 53). In the frog's skin acetyl- 
choline does not effect the time course of excitation 
or recovery but does lower the threshold and increase 
the rate of adaptation (53). The most likely explana- 
tion of the action of this substance is that it depolarizes 
the membrane of the terminal portions of the sensory 
nerve fiber and that this action is confined to those 
parts that take part in the generation of the receptor 
potential. This conclusion might lead one to suppose 
that acetylcholine plays some part in the normal re- 
sponse to a physiological stimulus. This, however, 
seems very doul)tful in the light of results obtained 
with blocking agents and anticholinesterases. 

Action oj Blocking Agents and Anticholinesterases 

It has been stated above that the action of acetyl- 
choline on sensory receptors is blocked by curare. It 
is also blocked by hexamethonium (17, 23), and large 
doses of nicotine (13). While these substances block 
the action of a subsequent dose of acetylcholine or 
nicotine, they have no effect, in most preparations, on 
the normal response to a physiological stimulus. Thus 
the mechanical receptors of the carotid sinus of the 



140 



HANDBOOK OF PHYSIOLOGY ^-^ NEUROPHYSIOLOGY I 



iliiiiM* 



mmi 




\ mmmmmmmmi \ m \ mmm i\ m \ M 



FIG. 12. Acetylcholine excitation of pressure receptors in the cat's carotid sinus, a: pressure in 
sinus, 25 mm Hg; injection of 0.5 ml saline, b: same pressure; injection of 0.5 ml of io~' g per ml 
acetylcholine, c: pressure, iii mm Hg; i.o ml saline, d: same pressure; 1.0 ml acetylcholine 10 » 
g per ml. Time, o. i sec. All records made 95 sec. after injection. [From Diamond (17).] 



cat are still able to produce a normal frequency- 
pressure curve when perfused with i per cent hexa- 
methonium (fig. 13), although the acetylcholine effect 
is lilocked lay a concentration of io~^ he.xamethonium 
(17). In the case of the carotid body, the chemical 
receptors of which appear particularly sensitive to 
acetylcholine, large doses of blocking agents diminisn 
the response to low oxygen tensions (61). 

Physostigmine does not affect the response of 
mechanical receptors in cat's and dog's skin to me- 
chanical stimulation (13), nor does it alter the 
pressure-frequency relationship of the pressure re- 
ceptors of the cat's carotid sinus (17). In two types of 
chemical receptor, anticholinesterases do enhance the 
response to the physiological stimulus; thus physo- 
stigmine and prostigmine increase the activity of 
chemical receptors of the cat's carotid sinus and 
prostigmine increases that of chemical receptors in 
the cat's tongue. 

These results suggest that acetylcholine cannot be 
an intermediary in the normal process of excitation 
at many types of receptor. Against this evidence, it 
has been argued that the blocking agents do not have 
access to the critical region; however, all these agents 
block the acetylcholine effect and nicotine is both an 
exciting and blocking agent. Such arguments can only 
be valid if it is argued that there is a third region on 
the sensory pathway that differs from the main part 
of the neuron in its sensitivity to these substances and 
from the receptor region in that it is not involved in 
the production of receptor potentials. There is no 
evidence that acetylcholine is present in receptors 
(13), but there is evidence of the presence of cholin- 



esterase in the Pacinian corpuscle (8, 43) and Meiss- 
ner's corpuscle (8); in the former this appears to be all 
pseudocholinesterase and its destruction does not 
appear to effect function in any way during an acute 
experiment (Diamond, J. & J. A. B. Gray, unpub- 
lished obser\ationsJi. 

The arguments against the participation of acetyl- 
choline as an intermediary in the normal process of 
excitation of some types of receptor do not exclude 
the possibility that local concentrations of acetyl- 
choline may modify the excitability of receptors under 
physiological conditions. There is no evidence for 
such an action of acetylcholine, but there is evidence 
that a parallel action can occur with epinephrine. 

Effects 0/ Sympathetic Stimulation and Epmep/inne 

Stimulation of the sympathetic supply to the skin of 
the frog has been shown to increase the excitability of 
the cutaneous receptors (67). Stimulation of the sym- 
pathetic in these preparations increases the response 
to a standard mechanical stimulus applied to the skin 
surface; also if the skin is stretched biu not otherwise 
stimulated mechanically so that there is no discharge 
in the aflferent fibers, stimulation of the sympathetic 
may initiate a discharge. These results are paralleled 
by the application of epinephrine to the skin. The 
effects of epinephrine and sympathetic stimulation 
add to those of mechanical stimulation of the skin 
and the application of currents to it. These results have 
been obtained in preparations which have been sub- 
.sequently sectioned and shown to contain no smooth 
muscle except that associated with the blood vessels 



INITIATION OF IMPULSES AT RECEPTORS 



141 



40 



30 - 



X 
o • 



20 



10 



_ £ 



•o 

X 



60 



-•-i- 



100 



-o-»J 



J_ 



_L 



_L 



220 



260 



140 180 

Pressure, mm Hg 

FIG. 13. Effects of hexamethonium on the pressure-response 
relationship of cat's carotid sinus receptors. Abscissa/ Pressure 
in sinus in mm Hg. Ordinate: impulse frequency per sec. O, 
X normal curves, • perfusion with i per cent hexamethonium. 
[From Diamond (17).] 



and they appear to be due to a direct effect of epi- 
nephrine on the receptor. Epinephrine can also in- 
crease the .size of the receptor potential of the Pacinian 
corpuscle in response to a given stimulus; this results 
in a lowering of the threshold (6g). In the carotid 
sinus of the cat there is also an effect of epinephrine, 
but in this instance the effect appears to be secondary 
to its action on the muscle of the sinus (17, 63). 

These results show that the activity of receptors 
may be modified by centrifugal activity. The idea 
is not, of course, new because the effects of stimulating 
the efferent fibers to the muscle spindles are well 
known (59). Centrifugal influences on the activity of 
the ear (29) and eye (31) are also under investigation, 
but whether or not these operate at receptor level is 
not yet clear. This topic is discussed also by Livingston 
(Chapter XXXI) on central effects on afferent activ- 
ity in this work. 

Othn Substances 

Histamine is a substance that has been much inves- 
tigated in relation to receptors, especially those con- 



cerned with the sensation of pain in man. Discussion 
of this problem belongs to another chapter. Many 
other agents have also been investigated (80) and 
special mention should be made of the sensitization of 
receptors by anesthetics (88, loi). 



MINUTE STRUCTURE OF RECEPTORS 

Electronmicroscopical studies have begun to throw 
some light on those structural relationships that mav 
be of importance in explaining the genesis of the re- 
ceptor potential in mechanical receptors. Sections of 
muscle spindles from the frog and of Pacinian cor- 
puscles from the cat's mesentery have been investi- 
gated. 

In the muscle spindle the finer branches of the af- 
ferent filler which are nonmedullated lie in close rela- 
tion to the intrafusal muscle fiber. These fibers, as they 
approach their termination, lose their Schwann cell 
sheath and come into direct contact with the muscle 
fibers; the continuation of the Schwann cell also runs 
in contact with the muscle but is separated from the 
axon. Smaller axons, which may represent the final 
terminations, are also seen in close relation to, but 
not in contact with, the muscle surface. The terminal 
parts of the afferent fibers contain many mito- 
chondria, though with no apparent orientation 
(fig. 14.4) (87). 

In the Pacinian corpuscle the axon is nonmyeli- 
nated from the point at which it enters the central 
core (86). At this point it has a diameter of 2 /x which 
it maintains until it ends. Over the whole of this 
nonmyelinated section there are certain characteristic 
features (85) (fig. 145). There appears to be no 
Schwann cell sheath; there are numerous mitochon- 
dria inside the nerve fiber arranged as a palisade 
around the fiber just beneath its surface membrane. 
The axon itself is not round but an ellipse in cross 
section and is surrounded by a complex cellular 
structure. This cellular structure is divided into two 
D-shaped parts separated from each other, in the 
middle by the axon, and on either side by gaps that 
continue the plane of the long axis of the elliptical 
nerve fiber. 

At this stage of such investigations, the most striking 
feature of the.se results is that both types of mechanical 
receptors show the terminal axon without a Schwann 
cell sheath. 



142 



HANDBOOK OF l'H%SIOLOGY 



NEUROPHYSIOLOGY I 



inner sh 




FIG. 14A. Diagram of a cross section of a portion of a frog's 
muscle spindle, at resting length, in the region of the sensory 
innervation. Inner sh., intrafusal muscle fiber inner sheath; 
m.nuc, muscle nuclei; mf., myofilaments; sarc, sarcoplasm; 
m., mitochondria; peri, subst., perimuscular substance; ax., 
axons; Sch., Schwann cells. [By courtesy of J. D. Robertson.] 




FIG. 14B. Diagram of a transverse section of the central core 
of a Pacinian corpuscle based on electronmicrographs. [By 
courtesy of A. Quilliam.] 



HYPOTHESES CONCERNING THE MECHANISMS 
OF RECEPTORS 

Many of our present ideas on the mechanisms in- 
volved in the initiation of impulses by receptors stem 
from the idea of nerve as a model sense organ (9). 
This concept invokes two parts; first that a constant 
current would excite repetitive discharges in a nerve 
fiiaer, and secondly that such currents are produced 
in nerves under physiological conditions by the devel- 
opment of generator potentials in the receptors. 

It is now known that many receptors produce recep- 
tor potentials and it is probably safe to assume that 
this is a generalization that applies widely. There is 
good evidence, which has already been considered, 
that these receptor potentials are the immediate cause 
of the impulse discharges. At present there is no evi- 
dence or need to suppose that the part of the afferent 
fiber in which the impulses are .set up differs from 
other parts of nerve fibers in its response to a flow of 
current, whether this be a flow of current due to a 
receptor potential, to an external source or to the 
summated effects of both. There is evidence that has 
already been considered which indicates that in the 
Pacinian corpuscle impulses are set up at the first 
node of Ransier and that the terminal nonmyelinated 
portion of the nerve fiber does not appear capable of 
conducting impulses. Similar conclusions can be 
drawn for the stretch receptor of the crayfish, though 
in this instance it is not possible to put such clear ana- 
tomical limits to impulse conduction. It may well be 
a general property of receptors that impulses are set up 
at a point central to the sensitive terminals by currents 
which are generated elsewhere. The summation noted 
between natural stimuli and externally applied cur- 
rents would result from a passive summation of the 
discharging process in this region of the membrane. 

The results just considered further suggest that the 
part of the nerve fiber that is unable to conduct a nerve 
impulse is the site at which the receptor potential is 
generated. This view is supported by the fact that the 
conditions under w hich receptor potentials have been 
recorded from the three mechanical receptors indi- 
cate that the current must have crossed the nerse fiber 
membrane peripheral to the point of recording. Esti- 
mates of the absolute value of the maximum receptor 
potentials suggest that it is unlikely that those currents 
that traverse the membrane of the nerve terminal are 
secondary to acti\ity in an external source. Further- 
more the fine structure of these terminals shows certain 
distinctive features. Thus, to take the specific example 
of the Pacinian corpuscle, the nonmyelinated terminal 



INITIATION OF IMPULSES AT RECEPTORS 



143 



appears unable to conduct impulses and this same 
region is structurally specialized, in particular in not 
having a Schwann cell sheath. The estimated poten- 
tial change that occurs across the membrane of this 
part of the fiber during a maximum receptor potential 
is of the same order of magnitude as the resting and 
action potentials. Many receptors are sensitive to 
acetylcholine, though it is not known whether or not 
the Pacinian corpuscle is sensitive. It is tempting to 
suggest that the inability to conduct impulses, the 
sensitivity to acetylcholine, the ability to produce re- 
ceptor potentials and the absence of the Schwann cell 
sheath are all connected. There is not howe\er enough 
evidence at present to support such an assertion. 

Olfactory receptors appear to fall in line with much 
of what has been said in the last few paragraphs. To 
some extent photoreceptors may as well, but these 
considerations belong to other chapters. For the rest 
of this discussion consideration will be given almost 
entirely to simple mechanical receptors as it is from 
receptors of this type that the relevant evidence is at 
present available. 

The next point to be considered is the immediate 
source of energy utilized in the production of a recep- 
tor potential. Maintained receptor potentials that last 
for minutes have been recorded, and if it is assumed 
that receptor potentials are responsible for the initia- 
tion of impulses in certain other receptors, for example 
the mechanical receptors in the carotid sinus of the 
cat, receptor potentials must remain constant for 
hours (17). Such potentials cannot be maintained 
across a biological membrane without the continual 
utilization of energy; such energy clearly cannot be 
provided by the work done during the deformation of 
the receptor. Since this is so, an internal store of 
energy must be available. It has already been argued 
that receptor potentials are generated across the mem- 
brane of the terminal portions of the afferent nerve 
fiber. Across this membrane is a store of energy in the 
form of the electrochemical gradients of the principal 
ions. It seems likely that it is this energy which is 
utilized during the activity of the receptor. In all 
slowly adapting receptors some such internal store of 
energy must be available; this does not necessarily 
follow for rapidly adapting processes such as the re- 
ceptor potential of the Pacinian corpuscle and the 
dynamic phase of the muscle spindle potential. While 
it seems reasonable that all mechanical receptors of 
the relatively simple group under consideration should 
have fundamentally the same mechanism, there is no 
conclusive evidence that this is so. In fact it has been 
suggested (58) that the static and dynamic phases of 



the muscle spindle receptor potential may have dif- 
ferent mechanisms. 

During receptor activity the potential across this 
membrane must alter. One suggestion discussed as an 
explanation of the dynamic phase of the muscle 
spindle receptor potential was that the potential 
change was a result of a change of membrane capacity, 
the total charge remaining constant; it was, however, 
pointed out that there were quantitative difficulties in 
this explanation (58). Similar calculations for the 
Pacinian corpuscle demand large increases in surface 
area which are known not to occur. A inore likely 
explanation of receptor activity is that ions transfer 
charge across the membrane by moving down their 
electrochemical gradients as a result of changes in the 
permeability of the membrane to one or more ion 
species (37, 58). If charge is to be transferred in such a 
direction as to explain the observed potential changes, 
cations must enter the fiber or anions leave it. The 
internal anions of nerve fibers are mostly large and 
less likely to move than the external cations which are 
almost entirely .sodium. If the mechanism in question 
were something of the kind suggested, it would then be 
expected that the receptor potential would be nearly 
abolished in the absence of sodium. This is in fact 
what has been observed in the Pacinian corpuscle. 
Another observation that can be explained on this 
hypothesis is that the rate of rise of the receptor poten- 
tial continues to increase with increasing stimulus 
strength at a level of stimulus strength at which the 
amplitude of the potential change remains practically 
constant. This can be explained by assuming that the 
permeability of the membrane continues to increase, 
so increasing the rate at which charge is transferred, 
while the final potential reached is limited by the 
equiliijrium potentials of the ionic gradients con- 
cerned. 

If the hypothesis put forward be accepted for the 
time being, the next problem is to consider how the 
changes of membrane permeability are brought about. 
This might be due to a distortion of the membrane or 
displacements in relation to surrounding structures, 
it might be due to a change of pressure in and around 
the axon or there may be chemical intermediaries 
outside or inside the axon. The last alternative still 
leaves the problem of how the mechanical energy 
produces the chemical intermediaries. At present there 
are no grounds for choosing between these mecha- 
nisms. However, if there are chemical intermediaries in 
the Pacinian corpuscle, the time course of their action 
(the latency often being less than 0.2 msec.) and their 
ability to function at room temperature (37) show 



'44 



HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



that they have quite different properties from main- 
iiialian synaptic transmitters. Other e\idence has been 
presented in an earlier section which suggests that at 
many mechanical receptors acetylcholine does not act 
as a chemical transmitter. 

The adaptation of receptors is a subject that has 
stimulated many hypotheses (47, 74). Until recently 
these have been based mostly on the concept that 
the adaptation of receptors is closely related to the 
accommodation of nerve fibers. It would certainly 
be expected that this factor would play a part if 
impulses are set up in the nerve fiber as a result of 
currents generated by receptor activity in the ter- 
minals. This factor cannot be entirely discounted, but 
there are now very good reasons for supposing that 
other factors may be more important. The time 
courses of all the receptor potentials so far observed 
are in general agreement with the corresponding 
time courses of the impulse discharge. Thus the 
short receptor potential of the Pacinian corpuscle 
corresponds to the single impulse produced by rela- 
tively large stimuli, the dynamic and static phases of 
the receptor potential of the muscle spindle corre- 
spond to the initial high frequency burst and the 
maintained discharge of impulses and the receptor 
potentials found in the two types of stretch receptor 
investigated in the crayfish correspond to the fast 
and slow adaptation of the two endings. The adapta- 
tion of the receptor potential inay simply reflect 
changes in the mechanical events going on in the 



terminals. This seems to ije the case in the Pacinian 
corpuscle where only a brief wave of distortion can 
be found in the central core during a maintained 
deformation of the outside of the endorgan. In the 
crustacean stretch receptors, the difference between 
the slow and fast cells has been attributed to differ- 
ences in the mechanical attachments between the 
dendrites of the two types of cell and the muscle 
fibers in which they ramify (27). The change in the 
rate of adaptation of receptors in frog skin, when the 
skin is stretched, is another example of the importance 
of mechanical factors. It is impossible to say whether 
or not such factors can account for the whole phe- 
nomenon of adaptation of the receptor potential. It 
is possible that there may be some mechanism that 
reduces the effectiveness of a stimulus as time passes; 
such a mechanism might conceivably be related to 
the depression of the receptor potential observed in 
the Pacinian corpuscle. 

Many of our ideas on the mechanisms of receptors 
are at the present time speculative. Definite ideas on 
these problems may develop as work goes deeper 
into the mechanisms of those few receptors which are 
particularly well adapted for such investigations. Also 
when results, that ha\e already been obtained on 
some receptors, are repeated or contradicted by 
work on other types, it may be possible to say how- 
far we may generalize from such results as have been 
obtained. 



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CHAPTER V 



Synaptic and ephaptic transmission 



HARRY GRUNDFEST 



Department of Neurology, College of Physicians and Surgeons, 
Columbia University, New York City 



CHAPTER CONTENTS 

Nature of Postsynaptic Potentials 

Generation Sites of Postsynaptic Potentials 

Molecular Structures of Differently Excitable Membranes 

Types of Postsynaptic Potentials 

Interrelations of Postsynaptic Potentials and Spikes 
Specific Properties of Synaptic Electrogenesis 

Evidence Against Electrical Stimulation of Postsynaptic Po- 
tentials 

Mechanisms of Bioelectrogenesis 

Other Consequences of Electrical Inexcitability 

a) Sustained electrogenesis 

b) Postsynaptic potentials during hyperpolarization and 
depolarization 

c) Electrochemical gradation and reversal of postsynaptic 
potentials 

d) Latency of postsynaptic potentials 

e) Electrotonic effects of presynaptic impulse upon post- 
synaptic region 

f) Chemical sensitivity of synaptic membrane 
Postsynaptic Potentials as Nonpropagated Standing' Po- 
tentials 

Interaction of Graded Responses 
Events in Synaptic Transmission 

Functional Interrelations Within Single Cell 

Evolution of Electrogenic Membrane 

Transmitter Actions 

Genesis of Postsynaptic Potentials 

Gradation of Postsynaptic Potentials 

Mechanisms of Graded Responsiveness 

Transfer of Activity from Postsynaptic Potentials to Elec- 
trically Excitable Membrane 

Synaptic Delay 

Superposition of Postsynaptic Potentials and Spikes 
General and Comparative Physiology of Synapses 

Forms and Magnitudes of Postsynaptic Potentials 



' The researches at the author's laboratory were supported 
in part by funds from the following sources : Muscular Dys- 
trophy Associations of America, National Institutes of Health 
(B-389 C), National Science Foundation and United Cerebral 
Palsy Associations. 



Cells with Depolarizing Postsynaptic Po- 
Hyperpolarizing Postsynaptic 



vith 



Postjunctional 
tentials 

Postjunctional 
Potentials 

Postjunctional Cells with Both Types of Postsynaptic Po- 
tentials 

Fast and Slow Responses of Invertebrate Muscles 
Pharmacological Properties of Synapses and their Physiological 
Consequences 

Classification of Drug Actions 

Identification and Characterization of Transmitter Agents 

Modes of Action of Transmitter Agents and Synaptic Drugs 

Physiological Implications 

a) Topographic distinctions 

b) Synaptic specificity and transmitters 

c) Reciprocal interactions of neural pathways 

Role of Elementary Synaptic Properties in Integrative Activity 
Spatial Interrelations of Synaptic and Conductile Membrane 
Physiological Factors Determining Transmissional Effective- 
ness 

a) Synaptic potency and drive 

b) Excited and discharged zones 

c) Facilitation 

d) Homosynaptic facilitation 

e) Heterosynaptic facilitation 

f) Spatial summation of converging pathways 
Integrative Utility of Electrical Inexcitability 
Synaptic Determinants of Different Types of Reflexes 
Role of Inhibition in Central Nervous System 
Physiological Effects of Different Porportions of Depolarizing 

and Hyperpolarizing Postsynaptic Potentials 
Synaptic Activity and Electrical Concomitants 

a) Interpretations of changes in amplitudes of postsynap- 
tic potentials 

b) Interpretation of electrotonic effects of standing post- 
synaptic potentials 

c) Synaptic transducer action and electrogenesis 
Ephaptic Excitation 

Electrical Modes of Transmission 
Role of Field Currents in Central Nervous System 
Dorsal Root Reflex 

Ephaptic Transmission in Annelid and Crustacean Nerve 
Cords 
a) Unpolarized ephaptic junctions 



■47 



148 



HANDBOC1K OF PHYSIOLOGY 



NEUROPHYSIOLOGY 



b) Polarized ephaptic transmission 
Evolutionary Aspects of Ephaptic Transmission 
Quasiartificial Synapses 



CONTRACTION OF A MUSCLE when an apparently un- 
reactive nerve is stimulated, the problem of trans- 
mission in its most obvious form, challenged the in- 
genuitv of early physiologists. Electricity quickly 
became a relatively familiar force after the invention 
of the Leyden jar and was inxoked in Galvani's 
theory (84). Electric fluid supplied from the central 
nervous system, he said, charged the interior of a 
muscle as the Leyden jar is charged by an electro- 
static machine. Contraction was cau,sed by discharge 
of this electrical fluid when the mu.scle and its nerve 
were connected by a metallic arc. The 'discharge 
hvpothesis' formulated by Krause and Kiihne in 
the i86o's encompassed as well the data obtained 
in the two decades after the foundation of electro- 
physiology by du Bois-Reymond and others. "A 
nerve onlv throws a muscle into contraction by 
means of its currents of action," said Kiihne in his 
Croonian Lecture of 1888 (133). This electric theory 
of transmis.sion (fig. i) was dominant until very recent 
times (98) despite the questions and doubts raised by 
du Bois-Reymond himself in 1874 (55), and by Bern- 
stein in 1882 (20). The former suggested that another 
inechanism, secretion by the nerve of some chemical 
agent, might be the cause of neuromuscular excita- 
tion. 

Transmission in the central nervous system hardly 
off"ered a problem to the physiologists of the nine- 
teenth century, chiefly for one reason. Nerve and 
muscle are distinctly diff"erent tissues performing 
different functions and obviously joined together at a 
specialized region, the endplate. Connections between 
nerve cells, however, were thought to be continuous, 
the neurofibrils of one penetrating into the body of 
another. This reticular theory of Gerlach was chal- 
lenged only at the end of the nineteenth century when 
His, Kolliker and pre-eminently Ramon y Cajal 
proposed the neuron theory (169), so named by VVald- 
eyer. Sherrington, in 1897 (181), applied the term 
synapse to the region of contact or contiguity at which 
transmission takes place from the presynaptic nerve 
cell to another, the postsynaptic cell. The present 
chapter will use these terms in their general context, 
including in this sense the neuromuscular and neuro- 
glandular junctions. 

The occurrence of demonstrai)le barriers at the 
contacts between neurons, different staining qualities 



B 





-_-:- A£ .-,-._, 



■ ■ ^IV^'") i^™^'^ il^^^i) * ' 



FIG. I. Models for electrical transmission. .-1, B. du Bois- 
Reymond's 'modified discharge hypothesis' of 1874 for the 
neuromuscular- junction. A: The current loops produced at a 
large endplate surface, which is itself not part of the muscle 
fiber, he thought would cause both anodal and cathodal de- 
polarizations. The current fields, indicated by the arrows, 
would thus alternate between excitant and depressant actions. 
B: du Bois-Reymond suggested that a geometrical arrangement 
which excited the muscle at a point contact would be more 
effective. [From du Bois-Reymond (55).] C.- Eccles' model of 
1946 proposed an essentially similar arrangement. Before the 
impulse of the presynaptic fiber had arri\ed at the synapse 
(left), there would be a hyperpol arizing (inward) current flow 
in the synaptic membrane. When the impulse reached its 
terminus (right) it would cause depolarization and excitation. 
[From Eccles (57).] D: Electrical model for inhibitory synaptic 
effects showing interaction of excitatory (E) and inhibitory (I) 
synapses. The latter were assumed to be the terminals of a short 
axon, Golgi II cell which developed a nonpropagating spike at 
its soma. The anodal focus caused by the I knob was supposed 
to depress the cathodal excitatory effects of the E knobs. Cur- 
rent flows are simplified in the diagram, loops which are sup- 
posed to diminish their excitatory effect are shown only at the 
edges of each E knob. [From Brooks et at. (28).] 



that indicate histochemical differences between pre- 
and postsynaptic units and the independent existence 
of the latter after destruction of the former (i.e. 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



■49 



absence of transneuronal degeneration) constituted 
the evidence brought forward by Ramon y Cajal and 
others in support of the neuron theory. 

When the neuron theory became accepted, the 
electrical theory of transmission, essentially as formu- 
lated by Kiihne for the neuromuscular synapse, was 
also generally adopted (cf. 45, 57, 140). Nevertheless, 
Sherrington's life-long study of the central nervous 
system emphasized that the physiological actions of 
the latter were dominated by the properties of syn- 
aptic transmission. These, he thought (183), were in 
many respects fundamentally different from the 
properties of conductile activity of nerve or muscle 
fibers, in which all-or-none impulses, spikes, are 
propagated by electrical local circuit excitation 
within the confines of a single cell, even though the 
latter may be very long in extent. A Russian school of 
physiology headed by Ukhtom.sky (cf i 76) ahso main- 
tained that central nervous phenomena could not be 
explained solely in terms of all-or-none activity. 

The neuron theory incorporates and gives physio- 
logical meaning to the doctrine of polarized conduc- 
tion which is embodied in the Bell-Magendie Law. 
The presynaptic terminals impinge upon the synaptic, 
or subsynaptic (cf. 60) membrane of the postjunctional 
cell with various types of contacts. These are located 
chiefly, but not exclusively, at the dendrites and soma 
of neurons, and Ramon y Cajal distinguished the 
different sites of contact as axodendritic and axo- 
somatic synapses (cf. 1^9). Contacts between the 
nerve fibers and the effector cells, muscle or gland, 
are also made at specialized regions, tho.se of mus- 
cle fibers being termed endplates, as noted above. 
Impulses afferent in a prefiber evoke activity in the 
postjunctional cell. If the cell is a neuron, its junc- 
tional activity may result in a spike which propa- 
gates along the latter's axon. At the terminals of this 
axon, a new transfer may then take place to another 
neuron or to an effector cell. In some instances 
unidirectional progression is apparently invalidated, 
but the general mechanism of these cases is probably 
b\ ephaptic transmission (10). This appears to be 
fundamentally different from synaptic transmission 
and will be discussed in the last section of this chap- 
ter. One recently discovered case of unidirectional 
conduction (83) produced by an electrical local cir- 
cuit mechanism will also be discussed at that time. 

The concept of unidirectional synaptic transmission 
permitted Ramon y Cajal to deduce many functional 
properties of the central nervous system from anatom- 
ical data (168). Changes that occur in gross and fine 
structure, in histochemical properties and in physio- 



logical behavior after extirpation or damage of 
specific elements also give clues to function. The 
information obtained by these methods relates chiefly, 
however, to the study of integrative activity which is 
the subject of later chapters. 

While it, too, bears largely on integrative functions, 
the analysis of reflexes as exemplified in Sherrington's 
work (cf 44, 182) nevertheless also provides data on 
the synaptic processes them.selves and discloses 
phenomena such as cumulative, long-lasting excita- 
tory and inhibitory slates. These two synaptic prop- 
erties endow the central nervous system with its 
remarkable flexibility and variety of responsiveness. 
Both characteristics may also be present in simpler 
peripheral synaptic organizations and are commonly 
found in the peripheral synaptic structures of inverte- 
brates. Sherrington's basic method, stimulation of 
selected pathways and study of their effects and inter- 
actions, has been refined by application of modern 
electrophysiological techniques. The combination 
has given information on the effects of different syn- 
aptic inflows, their relative potencies, the temporal 
and spatial distribution of excitatory and inhibitory 
actions, particularly in the spinal cord (cf. 140; and 
later chapters in this volume). 

The electrophysiological study of single unit path- 
ways such as nerve-muscle or neuron-neuron provides 
still more detailed and intimate information on synap- 
tic mechanisms (cf. 62). Microelectrode recording, 
either from the vicinity of single cells or from their 
interior, is a recent extension of the technique which 
can provide the most definitive information (52, 59, 
60, 95, 97). In all cases, transmissional activity is 
found to be associated with a special type of electrical 
response, the postsynaptic potential or p.s.p. The 
transmissional electrogenesis at the endplates of 
skeletal muscle fibers is known as the endplate po- 
tential (e.p.p.). Basically, however, the properties of 
e.p.p.'s arc identical with those of p.s.p.'s. A presyn- 
aptic potential, occurring at the terminals of dorsal 
root fibers, has also been described ijut from indirect 
evidence only (140). 

Pharmacological data provide much of the oldest 
evidence that synaptic transmis.sion is different from 
the conductile process. Claude Bernard (18) found 
that curare, the Indian arrow poison, blocked excita- 
tion of a muscle by its nerve. The muscle and nerve 
individually retain their conductile properties, and 
the primary effect of the drug is on the transmission 
process. Attempts to account for the synaptic blockade 
in terms of electrical transmission were not successful 
(cf. 98). A host of other chemicals exert actions chiefly 



I50 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



wherever synapses occur. These junctional regions 
also appear to have special biochemical requirements. 
Transmission, for example, is more easily disrupted 
by anoxia than is conduction. Pharmacological and 
biochemical tools, particularly in combination with 
the techniques of electrophysiology, provide additional 
data on the processes of synaptic transmission (cf. 
96, 99-101, 161-166). 

A challenge to the electrical theory was offered by 
that of chemical transmission which evolved chiefly 
from the work of Dale, Loewi, Cannon and their 
associates (cf. 150, 177). According to this view, 
activity of a presynaptic fiber releases at its synaptic 
terminals a chemical transmitter agent. That sub- 
stance excites the electrical activity of the postjunc- 
tional cell. By repetition of the secretory process at 
the terminals of the latter, a new action is started in 
the next unit of a transmissional chain. The present 
chapter adopts this view. 

The conclusion that synaptic transmission obliga- 
torily involves a chemical mediator derives from a 
hypothesis based upon a recent examination of data 
on available synaptic systems (97). All possess a com- 
mon constellation of properties that are shown in 
table I and discussed in the portion of this chapter 
devoted to synaptic electrogenesis. The entire group 
of these distinguishing characteristics appears to be 
referable to a single fundamental property of synaptic 
electrogenic membrane, namely that its activity is 
not initiated by an electrical stimulus. Thus, there 
arises a profound distinction between the conductile 
activity of axons or muscle fibers and the transmis- 
sional activity at synapses. The former is electrically 
excitable by an applied stimulus or by the internally 
generated local circuit of activity. The latter is elec- 
trically ine.xcitable and must be evoked by a specific 
stimulus which in the context of synaptic structure 
must be a chemical excitant, or transmitter agent, 
released by the active presynaptic nerve fibers. 

The currently used definition of synapses is still 
essentially as it developed with Sherrington and 
Ramon y Cajal, a junction in contiguity between 
anatomically distinct cells across which activity is 
nevertheless transmitted, but only in one direction, 
from the presynaptic cell to the postsynaptic. Many 
other specifications are now available to distinguish 
transmissional activity from conductile or ephaptic, 
and these appear to derive from the one feature, that 
synaptic activity is electrically inexcitable. 

N.-VTURE OF PDSTSYN.\PTIC PGTENTI.^LS 

The earlier studies of p.s.p.'s were made with 



external recordings from muscle endplates (59, 6q, 
63, 86), sympathetic ganglia (56) and the spinal cord 
(57' 58)- The muscle synapses being more easily 
accessible, it was most intensively studied both 
electrophysiologically and pharmacologically (cf. 
62). More recently, this and many other varieties of 
synapses have been investigated with intracellular 
recording (cf. 52, 59, 60, 68, 95, 97), and a reasonably 
coherent and satisfactory description of the principles 
of synaptic electrogenesis is now available. 

Generation Sites of Postsynaptic Potentials 

As noted above, p.s.p.'s are associated with the 
occurrence of transmissional activity at junctions 
between a pre- and a postunit. Only in a few systems 
(e.g. neuromuscular and squid giant axon synapses) 
is the junction confined to a clearly delineated area of 
the postunit. In these cases it is found that the p.s.p. 
is largest within the region of the junction and de- 
creases rapidly as the distance of the recording 
electrode from the junction increases (fig. 2). The 
form of the potential is also distorted in the manner 
characteristic of electrotonic spread (114, 141), both 
facts indicating that the site at which electrogenesis 
occurs is confined to the synaptic region. As will be 
described below, the nonpropagating, 'standing' 
response of p.s.p.'s is a consequence of electrical 
inexcitability. 

When the p.s.p. is recorded with a microelectrode, 
at first externally and then internally, the sign of the 
p.s.p. reverses when the electrode penetrates the cell. 
Like the spike, which also undergoes reversal of sign 
under the same conditions, the neurally evoked po- 
tential is produced at the excitable electrogenic 
membrane of the postjunctional cell, hence the term 
p.s.p. 

Molecular Structures of Differently Excitable Membranes 

The structures of the membranes that are involved 
in ssnaptic activity are not as yet known. The pre- 
synaptic terminals occur in an immense variety of 
shapes and sizes. In some of these electron microscopy 
has indicated the presence of vesicles (54, 174). The 
latter have been interpreted (cf. 52) as sites of concen- 
tration of chemical mediators which presumably are 
formed in the nerve fibers and ejected during activity 
into an extracellular synaptic space of about 100 A. 
The postsynaptic sites which respond specifically to 
the chemical transmitter agents cannot, at present, be 
differentiated structurally from those of electrically 
excitable membranes. This is perhaps l^est exemplified 



SYNAPTIC AND EPHAPTIC TRANSMISSION 




FIG. 2. Some properties of depolarizing postsynaptic poten- 
tials. .-1, B: The intracellularly recorded e.p.p. of a mammalian 
muscle fiber is evoked by neural stimuli during hyperpolariza- 
tion of the muscle fiber membrane through another intracellu- 
lar electrode. The impaled hyperpolarized fiber did not re- 
spond with a spike or contraction, but others unaffected by the 
polarizing current and excited by the neural \olley contracted. 
The resulting movement pulled the microelectrode out of the 
tested muscle fiber producing the artifact seen at the end of 
each record. The response in B is smaller than that in A, 
partly because it is generated at a less hyperpolaiized membrane 
as is described in the text. However, it is also broader than the 
response in A, indicating that the recording microelectrode was 
probably some distance from the focus of the e.p.p. The effects 
of recording at various distances from this focus are shown in 
C, D and E. The amplitude of the e.p.p. falls sharply (C); the 
rising phase is prolonged somewhat (i)) and the falling phase 
even more (£) as the electrode is moved farther from the focus. 
[From Boyd & Martin (23).] 



by electron microscopic studies of eel electroplaques 

(95). 

These cells possess three functionally distinct types 
of membrane. One major surface is composed of 
membrane that does not respond electrogenically to 
any type of stimulation and has a very low electrical 
resistance. The other major surface of each cell is 
diffusely innervated and, presumably only under the 
presynaptic terminals, there is excitable membrane 
of the synaptic type which responds only to neural or 
to chemical stimuli. Intermingled with this electri- 



cally inexcitable membrane component is one that is 
electrically excitable and produces a spike. Electron 
microscopy has as yet not been able to discern differ- 
ences between the two different components of the 
excitable membrane, nor between their structures and 
those of the nonresponsive membrane (95, 143). 

Two functionally quite different junctions, in squid 
and crayfish respectively, appear to be similar when 
observed by electron microscopy C'75)- However, 
that activating the giant axon of squid is electrically 
ine.xcitable and thus conforms to the extended defini- 
tion of synapses given above. On the other hand, the 
junction between a medial giant fiber and the motor 
giant axon of the crayfish (83), as will be discussed 
below, appears to resemble the ephaptic junctions of 
septate giant axons (125). 

The inai)ility of present day microscopic techniques 
to differentiate the structures of membranes which 
differ profoundly in their functional properties indi- 
cates that the differences which determine these 
properties must be at the molecular level. Probably, 
as microscopic methods develop, the difficulty of 
visualizing molecular differences will be overcome. 
At present, however, the chief tools available for 
analyzing these structures are electrophysiological 
obser\ations of function and of the disturbance in 
function produced by various experimental means, 
including the use of chemical agents (cf. gg-ioi; 163). 

Types of Postsynaptic Potentials 

•Synaptic electrogenesis differs from that of the 
spike by being relatively small and, when more than 
one nerve fiber is available to excite it, is graded in 
amplitude depending on the strength of the stimulus 
to the nerve. Furthermore, two varieties of p.s.p.'s can 
occur. One, like the spike, tends to decrease the resting 
potential, hence is a depolarizing p.s.p. The other 
tends to increase the resting potential and is therefore 
a hyperpolarizing p.s.p. The two varieties of p.s.p.'s 
are present in different proportions in different cells. 
Some cells generate only depolarizing, others only 
hyperpolarizing p.s.p.'s, while in a third group both 
types of responses are produced usually, and perhaps 
always, by stimulation of different neural inflows. All 
vertebrate muscle fibers thus far known, their em- 
bryological relatives the electroplaques of most elec- 
tric organs and some neurons develop only a depolar- 
izing p.s.p. Certain gland cells are at present known 
in which a hyperpolarizing p.s.p. is the sole electro- 
genesis (144, 146). The crayfish stretch receptor, 
likewise, produces a hyperpolarizing p.s.p. C'So), but 



152 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



depolarizing electrogenesis is also evoked, although in 
this case by stretch of the mechanosensory receptor 
membrane (66, 67, 94). In other cells, notably neurons 
of the vertebrate central nervous system (59, 60, 158, 
159, 1 61-16 7) and some invertebrate muscle fibers 
(73. 13O and neurons (33, 186), l:)oth types of p.s.p.'s 
are found. 

The depolarizing p.s.p., being of the same sign as 
the effective stimulus for electrical or local circuit 
production of a spike, can also evoke the latter and is 
therefore termed an " excitatory' p.s.p. (59, 60). The 
spike arises when the p.s.p. is sufficient to depolarize 
the adjacent electrically excitable, spike-generating 
membrane to a critical firing level (fig. 3). The latter 
varies among different cells and is of the order of 10 
to 40 mv change from the resting level. The hyper- 
polarizing p.s.p., by the same criterion, is an ' in- 
hibitorv' p.s.p. However, these names are not always 
appropriate. There are cells, like some electroplaques 
or muscle fibers (cf. 95, 97), that generate depolarizing 
p.s.p.'s but no spikes. The depolarizing p.s.p. there- 



fore may have nothing electrogenic to excite. Like- 
wise, those gland cells which generate only hyper- 
polarizing p.s.p.'s al.so have no spike to inhibit (cf. 
fig. 20). On the contrary, the hyperpolarizing electro- 
genesis of the gland cells is associated with actixity in 
the form of secretion (146). 

When the two varieties of p.s.p.'s occur in a cell 
\shich also generates spikes, they interplay with 
excitatory and inhiljitory influences upon the elec- 
trically excitable membrane. The inhibitory synaptic 
action may occur independently of the magnitude 
and even tiie sign of the inhibitory p.s.p. As will be 
descriljed below (p. 160) this p.s.p. may be de- 
polarizing under certain electrochemical conditions, 
or the acti\'ity of the synaptic membrane may not 
manifest itself as a potential. Nevertheless, when this 
synaptic activity is pitted against a depolarizing 
p.s.p. it always tends to decrease the magnitude of 
the latter and thereby to diminish or block its ex- 
citatory effect on the electrically excitaljle membrane. 
In some cases, therefore, the term "inhibitory" p.s.p 




msec 




msec 



FIG. 3. Synaptic transfer from the p.s.p. to the spike. Intracellulai" recording, eel electroplaque. 
Above: Increasing stimuU to a nerve produced a stepwise increase of the p.s.p. (.4 to C). A still larger 
stimulus evoked a spike (Z) and £). Below: The p.s.p. first generates a local, graded response of the 
electrically excitable spike-generating membrane. When the neural stimulus evokes a p.s.p. during 
the absolute refractory period {A', B'), the response lacks this component of giaded activity of the 
electrically excitable membrane. Later (C to G") the local response develops, grows, arises earlier 
and fuses with the p.s.p. The combined response is seen in isolation in H'. This series of records was 
taken at approximately ' 1 the amplification of the upper set. Baseline denotes the zero for the 
resting potential and for the overshoot of the spikes. [From Altamirano el al. (4).] 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



153 



is more apt than 'hyperpolarizing' p.s.p., btit the 
lattei term may be extended to denote a tendency 
to maintain as well as to increase the resting potential. 

Interrelations of Postsynaptic Potentials and Spikes 

It has been noted above that the p.s.p. is not ac- 
tively propagated as is the spike. Thus, the transmis- 
sional electrogenesis of a p.s.p. is confined to the syn- 
aptic site. While their local electrical activity can be 
recorded in or about the cells that produce it (cf. 
51, 70), p.s.p.'s do not, in general, evoke activit\' in 
other cells, their effects being confined to the cell in 
which they originate. 

To elicit 'distant' actions in the next postjtinctional 
cell, the prejunctional cell must generate a spike. 
Thus, transmissional activity in a synaptically linked 
chain of units is consummated only if the p.s.p. of each 
unit evokes a spike. When the depolarizing p.s.p. in 
one of the linked elements is insufficient to elicit a 
spike, the transmissional chain is broken. Likewise, if 
at one synaptic site, inhibitory p.s.p. is sufficiently 
large to block the spike of that cell, the chain is also 
broken. 

Thus, spikes and p.s.p.'s are functionally interre- 
lated. The former command the secretory activity at 
presynaptic terminals of their cell, and the released 
transmitter agent then evokes the p.s.p. of the next 
cell, which may or may not itself elicit a new spike, to 
repeat the process. It should be noted that while 
hyperpolarizing p.s.p.'s can inhibit spike production, 
they are themselves evoked by an excitatory activity 
in the presynaptic cell that propagates within the 
latter and effects the hyperpolarization of the post- 
junctional synaptic membrane through the secretory 
activity that it calls forth in the presynaptic terminals. 
In other words, a p.s.p., whether excitatory or 
inhibitory, always represents an active process, a 
response of subsynaptic membrane to an appropriate 
excitant. 

As was noted abo\e, and will be descriiied in more 
detail below, the electrically inexcitable synaptic 
electrogenic membrane has different properties from 
those which generate the spike. The properties even of 
simple synaptic s\stems are therefore compounded 
from and subject to the various properties of the differ- 
ent electrogenic components. The multiplicity of syn- 
aptic transfers in the central nervous system makes the 
synaptic properties a dominant factor, although those 
of conductile electrogenesis are also important. Since 
the amount and type of synaptic electrogenesis deter- 



mines the occurrence or absence of spikes, factors 
which modify p.s.p.'s are therefore of sjreat significance 
in the central nervous system. Among these are the 
effects of pharmacological agents or synaptic drugs, 
and their use as experimental tools has already been 
mentioned. However, other agents and physiological 
conditions may affect production of p.s.p.'s. For ex- 
ample, the synaptic membrane may be altered in its 
properties by previous activity (cf 95, 97; and below) 
and this could affect synaptic electrogenesis. The 
physiological properties of the presynaptic terminals 
may also be changed by various conditions, including 
previous activity. This change might affect the amount 
or nature of the transmitter agent released under the 
new circumstances and thereby aflfect transmission. 
Thus, the magnesium ion interferes with release of 
transmitter agents from the presynaptic terminals 
(cf. 52). Neuromuscular transmission is then depressed 
or blocked. The calcium ion acts reciprocally and in 
excess antagonizes the effects of excess magnesium 
ion. 



SPECIFIC PROPERTIES OF SYN.APTIC ELECTROGENESIS 

Evidence Against Electrical Stimulation of 
Postsynaptic Potentials 

The existence of varieties of postjunctional cells in 
which p.s.p.'s are generated without spikes, e.g. in 
Torpedo and Raia electroplaques, invertebrate and 
vertebrate muscle fibers and gland cells (cf. 95, 97), 
provides one kind of direct evidence for electrical 
inexcitability of synaptic membrane (figs. 4/I, 5; cf. 
fig. 20). An electrical stimulus which does not fire the 
presynaptic nerve fibers evokes no electrical activity 
in these cells. Responses are only produced by afferent 
neural activity or by chemicals which thus mimic the 
action of the transmitter agent (fig. 5). 

Even in those cells which also generate spikes, the 
p.s.p. is produced only by neural or chemical stimuli. 
Direct electrical stimuli applied to the cell, or its local 
circuit excitation by antidromic invasion, evoke only 
spikes without p.s.p.'s (fig. 6). Finally, the occurrence 
of spikes and of absolute refractoriness which is their 
concomitant does not preclude the independent 
development of p.s.p.'s. The electrogenesis of the 
latter then can be superimposed upon that of the 
spike, i.e. it can be evoked during the absolute refrac- 
tory period (figs. 6, 7). Together therefore, these three 
types of data provide direct evidence that the p.s.p.'s 
are generated by membrane that is not itself electri- 



'54 



HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 





^ 





TABLE I. Characteristics and Properties of Differently 
Excitable Electrogenic Membrane 



FIG. 4. Differences between electrically inexcitable and ex- 
citable membrane. A : The slow muscle fiber of the frog is not 
electrically excitable and produces no spikes, even when the 
membrane is strongly depolarized at beginning of (a). It 
develops p.s.p. on stimulation of the nerve during the applica- 
tion of the electrical pulse. The response at the resting potential 
(«), a depolarizing p.s.p., is increased when the membrane is 
hyperpolarized by the applied pulse (/, g). The p.s.p. is de- 
creased by depolarizing the membrane (</) and is reversed in 
direction by strongly depolarizing the membrane (a to f)- The 
magnitude of the reversed depolarizing p.s.p. increases as the 
interior of the membrane is driven beyond an equilibrium po- 
tential given approximately by the pulse in c. [From Burke & 
Ginsborg (35).] B: Responses of a cat motoneuron to ortho- 
dromic stimuli show essentially the same behavior of the p.s.p.'s, 
but are complicated by the appearance of a spike and the inac- 
tivation of electrically excitable membrane. The response at the 
resting potential ( — 66 mv) is a depolarizing p.s.p. which does 
not elicit a spike. Hyperpolarization of the membrane caused 
little change in the p.s.p. Depolarizations to —60 mv and —42 
mv summed with the excitatory effect of the p.s.p., evoking 
spikes. These are no longer produced by the p.s.p.'s at the rest- 
ing potential — 32 mv, etc. These depolarizations, after evoking 
spikes by the electrical stimuli, then inactivated the spike- 
generating membrane. The p.s.p.'s decreased and at a mem- 
brane potential of +3 mv disappeared but reappeared in 
reversed sign as the internal face of the membrane was made 
more positive. [From Eccles (60).] 

cally excitable. Other properties of p.s.p 's that dis- 
tinguish them from spikes are also referable to this 



Spike 
(Electrical^ Excitable) 



P.s.p.'s 

(Electrically Inexcitable) 



A. Characterisii 
Transducer action; 

(i) Sequential increase of 
Na^ and K+ conduct- 
ances and Na* inactiva- 
tion 



(ii) Rates determined by 
membrane potential 
Electrical response: 
(i) Begins with graded de- 
polarization, develops 
overshoot 
(ii) All-or-none response 



Two types: 

a) increased conductances 

for all ions 
A) specific increase in K+ 
and/or Cl~ conduct- 
ances 
Rates not determined by 
membrane potential 

Two types: 

a) depolarizing 

6) hyperpolarizing 
Graded response 



B. Direct Evidence Jor Characteristic Differences 

Developed only by neural or 



(i) Spike absent | 
(ii) Spike presentj 



chemical stimuli 



C. Consequences of Characteristic Differences 



(i) Always in depolarizing 
direction 

(ii) Hindered or blocked by 
hyperpolarization 

(iii) Excited, then blocked 
by depolarization 

(iv) Pulsatile, relatively 

fixed duration inde- 
pendent of stimulus 
(v) "Vanishingly brief la- 
tency 

(vi) Relatively inert to 
chemicals 



(vii) Decrementless propaga- 
tion 



Of either sign, electrochemi- 

cally reversible 
Electrochemical gradation 

Electrochemical gradation 

May be prolonged, sustained 
while stimulus lasts 

Appreciable, irreducible la- 
tency 

Sensitive in two ways: re- 
sponse may be 
a) evoked by synapse acti- 
vators; 
i) depressed or blocked 
by inactivators 

Nonpropagated, 'standing' 
potential 



single, fundamental difference in their modes of 
excitation. These correlations are summarized in table 
I, and form the content of this section (cf. also 97). 



Mechanisms of Bioelectrogenesis 

The means by which a cell can generate electrical 
activity are restricted in variety by the nature of the 
physiological and electrochemical systems of living 
tissues (91, 112, 113). Conductile and transmissional 




SYNAPTIC AND EPHAPTIC TRANSMISSION 1 55 

A - r\ B 



FIG. 5. Electrogenic action of acetylcholine on the elec- 
trically inexcitable membrane of Torpedo elcctroplaques. Intra- 
arterial injections of 10 /ng (/), 5 /ig (//) and 2.5 /ig (/!') in the 
presence of physostigmine produced electrical activity, the 
larger amounts evoking the larger responses. The neurally 
evoked discharge of Torpedo organ lasts only a few msec. (cf. 
95). The long duration of the response produced by injections 
of acetylcholine presumably is due to sustained depolarization 
of the electrically inexcitable elcctroplaques by an excess of the 
administered transmitter agent. /// indicates a control in which 
only perfusion fluid was injected. The elcctroplaques were 
probably depolarized in the 'resting' state, and the 'hyper- 
polarization' seen in this record may have been caused by 
temporary dilution of the depolarizing e.vcitant. Calibrations: 
0.5 mv, and seconds. [From Feldberg & Fessard (74).] 



excitable membranes utilize the electrical polarization 
or resting potential of the cell. This appears as a 
potential difference across the cell membrane with its 
interior negative relative to the exterior. At rest, the 
membrane has a rather high resistance, indicating 
that it presents a considerable barrier to the penetra- 
tion of ions. The physiological electrogcnic response 
of the membrane to an appropriate stimulus, its 
transducer action (94), is the temporary alteration of 
its permittivity to ions. The electrical change is its 
consequence, derived from the prior, metabolically 
energized unequal distribution of ions and the resting 
potential. 

Whereas the spike is generated by temporally 
sequential processes comprising first enhanced sodium 
conductance, then enhanced potassium conductance 
and sodium inactivation(ii3),^ the transducer actions 
of svnaptic membrane involve different ionic events. 

- Recent data on eel elcctroplaques (3) indicate that a process 
of potassium inactivation may be involved in spike production 
(95). The participation of other, potential-insensitive processes 
is discussed below in connection with graded responses of 
electrically excitable membrane. 




FIG. 6. Some differences between electrically and neurally 
excitable responses. A, B: Weak and strong depolarizing elec- 
trical stimuli to the eel electroplaque excited the cell directly, 
the latter with almost no latency. C, D: The stimuli were ap- 
plied in the reverse direction. These are ineffective for the 
electrically excitable membrane but stimulate the cell indirectly 
by way of the nerve terminals supplying the synaptic mem- 
brane. The weak indirect stimulus evoked only a p.s.p. after a 
latency of almost 2 msec. (C). The very strong stimulus (i)) 
shortened the latency to about 1.7 msec, and the larger p.s.p. 
evoked a spike with very brief delay. No p.s.p.'s were produced 
by the direct stimuli. However, the strong direct stimulus (B) 
also excited the nerve fibers which csokcd a p.s.p. that occurred 
with the same latency as in C and D but appearing this time on 
the falling phase of the directly elicited spike. The p.s.p. there- 
fore occurred while the electrically excitable membrane was 
absolutely refractory. [From Altamirano et al. (4).] 



Depolarizing p.s.p.'s are caused by a general increa.se 
of permittivity to all ions (71 ; cf. 52, 60) which tends 
to abolish the resting potential. Electrogenesis of 
hyperpolarizing p.s.p.'s probably involves increased 
permittivity for K+ and Cl^ (60, 61; Grundfest 
el al., in preparation). Each ion species then moves in 
the direction of its electrochemical gradient, K+ 
outward and Cl^ inward. Loss of positive charges and 
gain of negative thus account for the increased 
internal negativity. 

The immediate consequences of electrical inexcita- 
bility of synaptic transducer actions are made appar- 
ent by the diagram of figure 8. Depolarization is the 
stimulus that initiates transducer action of an elec- 
trically excitable membrane. The entry of Na""", 
forced inward because of the high concentration of 
this ion in the external medium, causes further de- 
polarization. This electrogenic response to the trans- 



156 HANDBOOK OF PHYSIOLOGY -^ NEUROPHYSIOLOGY I 



FIG. 7. Absence of refractoriness in postsyn- 
aptic responses in the giant neuron of Aplysia. 
A : A single shock to the presynaptic nerve first 
evokes a long-lasting p.s.p. out of which rises 
the spike of the giant neuron. B: A second stim- 
ulus, exciting the cell during its refractory pe- 
riod, adds a potential (solid line beginning at 
arrow) to the initial response (broken line). 
The difference (dotted line, below) is due to 
the second p.s.p. C: The second stimulus was 
delivered somewhat later. The added potential 
also shows a local response (prep.") which was 
initiated by the p.s.p. in the electrically ex- 
citable membrane during the relatively refrac- 
tory period. D: At a longer interval, a second 
stimulus evokes the full response as in A. [Froin 
Arvanitaki & Chalazonites (11).] 




B 




/>■»/» 



looms 



ducer action of the membrane can then act as a further 
stimulus to the latter. The positive feedback of the 
effect leads to a regenerative sequence and to the 
explosive, all-or-none spike. Since the transducer 
actions of electrically inexcitable membrane are not 
affected by the electrogenesis of the p.s.p.'s, feedback 
either positive (in the case of the depolarizing), or 
negative (for the hyperpolarizing p.s.p.'s) is lacking. 
Because of the absence of electrical feedback p.s.p.'s 
of either sign are thus produced that are graded in 
proportion to the availability of the specific excitants 
of the respective transducer actions. 

Ot/wr Consequences of Electrical Incxcitability 

a) susT.oiiNED ELECTROGENESIS. Thc transducer actions 
of the spike generator are a sequence of potential- 
determined events, the first (.sodium conductance) 
tending to cause the depolarizing electrogenesis, 
others (potassium conductance, sodium inactivation) 
tending to terminate it and to restore the resting 
potential. The sensitivity of these processes to the 
changes in membrane potential produced by the 
electrogenesis itself thus leads to a self-limiting event, 
the spike, of rather constant duration with which is 
also associated refractoriness (113). Not being elec- 
tricallv excitable, the transducer actions of the syn- 



aptic membrane are relatively in,sensitive to the 
changes of membrane potential. Hence, p.s.p.'s 
may be sustained as long as the excitant of the trans- 
ducer action is available (fig. g) since they are not 
subject to refractoriness (figs. 6, 7) nor inactivation. 
The transducers of most types of sensory membrane 
are probably also electrically inexcitable (94, 95, 97). 
The sustained graded electrogenesis which can de- 
velop to a sustained stimulus is the means for trans- 
mitting information by a train of pulsatile spikes, 
coded as to frequency and number in some relation 
to the intensity and duration of the stimulus (97, 103; 
fig. 10; cf. fig. 13). The transducers of some mechano- 
sensitive organs, at least, also have chemical sensitivity 
(94, 96, 97), indicating further their relations with 
chemically sensitive synaptic membrane. 

Although the postsynaptic membrane, in contrast 
to the electrically excitable, is capable of sustained 
electrogenesis, its responsiveness to a steady stimulus 
mav be affected in various ways. These reflect the 
labilitN of thc membrane in the face of the very chemi- 
cal agents by which it is excited (95, 96). An example 
is the gradual diminution or even disappearance of 
synaptic electrogenesis when a muscle or autonomic 
ganglion is continuously acted upon h\ acet\ Icholine 
or other agents (123, 127, 129, 187). 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



157 



Stimulus 


Transducer Action 

(Increosed Memtifor^e 
Conduclonce) 


Electrogenesis Response 


Electrical 

(Depolarizing) 


— No* -^ 


Depolarizotion — • Spike ond 

decrementless 








Chemicol 


► General — ► 


Depolonzotion — • Graded excitotory 
posl-synoptic 
potential 


Chemical 


► K+and/orCr — ► 


Hyper- — ► Groded inhibitory 
polorizotion post -synoptic 
potential 



FIG. 8. The different ionic mechanisms evoked by transducer 
actions in electrically excitable and synaptic membranes, and 
some consequences of the different excitabilities. The depolari- 
zation caused by an electrical stimulus is regenerative in the 
electrically excitable membrane and produces the all-or-none 
spike. The electrically inexcitable synaptic membrane can 
produce either depolarizing or hyperpolarizing p.s.p.'s which 
do not react back on the transducer actions. This insensitivity 
to electrical effects results in responses graded in proportion to 
the available chemical stimulus. The depolarizing p.s.p. can 
act as a stimulus for the electrically excitable membrane, while 
the hyperpolarizing is inhibitory to the latter. [From Grund- 
fest (96).] 



The kinetics of this reversible desensitization have 
been studied thus far only in frog muscle endplates 
(fig. 11). The nature of the processes involved C127) 
is not yet clear; but neither the loss of responsivene.ss 
nor its recovery are controlled by the membrane po- 
tential. 

Desensitization may be slow and unimportant 
relative to the excitatory events that occur at synapses 
in response to their normal neural activation. How 
ever, it might become a disturbing factor if trans- 
mitters are continuously released locally or svstem- 
ically. This situation could result from the action of 
drugs or might arise from a pathological state. Rapidly 
developing desensitization has not yet been described, 
but it might account for the successively decreased 
p.s.p.'s sometimes produced by a train of stimuli. 
This process has been termed 'defacilitation' (33, 
186). Decrease in the generator potential of sense 
organs acted upon by a constant stimulus, such as is 
seen in the rapidly adapting stretch receptors of cray- 
fish (66), might be accounted for by a desensitization 
phenomenon. 








100 msec 



i—r 



15 msec 
T M M 



FIG. g. Soine consequences of the differently excitable electrogenic mechanisms in neurons, a.' 
The cat motoneuron excited antidromically at high frequencies (140, 205, 280 and 630 per sec.) 
produces pulsatile spikes, only their after -potentials fusing. [From Brock et al. (25).] b: The p.s.p.'s 
produced by orthodromic stimuli (205 and 280 per sec.) summate, a higher average level of the de- 
polarization being produced by the higher frequency of stimulation. The summated response is 
maintained as long as the afferent stimuli are delivered (lower record of each set). The amplitude 
calibration applies to the p.s.p.'s of this .set which were taken at about lox the amplification of a. 
[From Brock et al. (24).] c: Repetitive activity evoked in the rabbit cervical syinpathetic neuron by 
stimulating the preganglionic supply at approximately 80, 100, 120 and 150 per sec. At the time 
scale of the records the first p.s.p. is not shown (cf. fig. 17C). The p.s.p. evokes a large spike; but 
even at the lowest frequency, the spikes caused by the subsequent p.s.p.'s are small, while the p.s.p.'s 
themselves are summed and sustained. This synaptic depolarization, increasing at higher frequencies 
of afferent drive, inactivates the spike-generating membrane. \i\.er the second depressed spike the 
responses progressively decrease, and at the highest frequency disappear. The p.s.p.'s are generated 
as long as there is an influx of presynaptic stimuli. [From Eccles (64).] 



158 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



lOmV T 




wUWWwWUUUWwwUUWUUUvi 



1 1 I 



! 





FIG. lo. Depolarizing electrogenesis of crayfish mechanoreceptor sense organ and the effects it 
evokes in the electrically excitable portion of the cell. Top: A weak stretch stimulus (I) caused a 
depolarization of about 7 mv across the membrane of the cell body. This was maintained until the 
stretch was released ( 1 ). Aliddle: Records at lower amplification. A weak stimulus produced a low 
frequency discharge of spikes. Increased stretch (second arrow) caused a higher frequency discharge 
which continued with some slowing as long as the stimulus was applied. The spikes generated during 
the depolarization develop a hyperpolarizing undershoot which is absent when the response is 
evoked by a single electrical stimulus. Bottom: Three increasingly larger stimulations are shown in A 
to C. The spikes produced at high frequency by the strongest stimulus (C) were diminished in ampli- 
tude and at the end were no longer evoked, while the receptor continued to respond with its sus- 
tained, summated depolarization. D to F: The return of responsiveness of the electrically excitable 
membrane after its inactivation. Note that the average level of the depolarization produced by the 
mechanoreceptor dendrites is graded with the degree of the stimulus. [From Eyzaguirre & KufHer 
(66).] 



b) P0STSVN.\PTIC POTENTI.ALS DURING HYPERPOL.^RIZ.^- 

TiON AND DEPOLARIZATION. P.s.p.'s Can bc produccd 
during hyperpolarization of the cell, while spike 
electrogenesis may be blocked (fig. 1 2). These differ- 
ent effects may be ascribed directly to the different 
modes of excitation of the electrogenic membrane 
components. The effects produced by depolarization 
are somewhat more complicated but can also be 
accounted for on the same basis. Superposition of 
depolarization by a brief extrinsic electrical stimulus 
and that of a depolarizing p.s.p. enhances the excita- 
tion of the electrically excitable membrane (4, 60, 
79). The spike thus arises earlier on the p.s.p. since 
the critical level of depolarization is thereby attained 
earlier. 



Sustained depolarization, in some cells even when 
rather small, blocks spike electrogenesis (fig. 13} 
probably (cf. 95, 96) by the augmentation of sodium 
inactivation and potassium conductance that it causes 
in electrically excitable membrane (113). Electrical 
inexcitaijility of synaptic transducer action permits 
the continued development of p.s.p.'s after the spike 
can no longer be produced by direct or neural stimuli 
(figs. II and 13). Other manifestations of synaptic 
activity can also be evoked when the spike generating 
membrane is inactivated by ionically induced depolar- 
ization (50). The generator potential of a sense organ 
(fig. 10) may also continue to be produced even 
though that sustained depolarization inactivates the 
electrically excitable membrane and no spikes can 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



159 




FIG. II. Desensitization of the synaptic membrane of frog sartorius muscle fibers by sustained 
applications of acetylcholine. The drug was applied through each of two pipettes close to the surface 
of the endplate. From one pipette it was released at regular intervals in brief jets of approximately 
constant quantity. These testing stimuli are signaled by dots on the lower line in each set. The upper 
line shows the response of the endplate recorded with an internal microelectrode. The e.p.p.'s in 
these records are compressed on the slow time scale. In the course of the recordings a larger longer- 
lasting jet of diflTerent amounts of acetylcholine was also applied to the endplate as a conditioning 
stimulus. Lejt: An otherwise normal preparation, a: The conditioning stimulus was a weak dose of 
acetylcholine applied for a long time, b to d: The concentration was higher, and the drug was applied 
for different times. The testing responses diminished progressively during the depolarization pro- 
duced by the conditioning stimulus. Their amplitudes recovered gradually after the conditioning 
depolarization had ended. Note that the recovery from desensitization is not associated with further 
change in potential. The recovery process therefore is not controlled by the membrane potential. 
Right: The muscle was immersed in isotonic potassium sulfate which depolarized the fibers and 
rendered them unresponsive to electrical stimuli. The tested muscle fiber was made inside-positive 
by about 15 mv by means of an intracellularly applied current. The synaptic membrane remains 
excitable to acetylcholine following these procedures, but the sign of the response is now reversed 
for reasons that will be discussed in the third subsection of this portion of this chapter. The membrane 
still exhibits desensitization to different intensities of the excitant drug (jop to bollom). The desensiti- 
zation process itself therefore is also not controlled by the membrane potential. At the end of the 
lower record the internal recording electrode was withdrawn from the muscle fiber (at the arrow}, the 
trace going from a level of internal positivity to that of the reference zero potential. [From Katz & 
Theslefr(i27).] 



develop. Thus, the sustained depolarization at 
sensory receptor terminals or at synaptic junctions, 
which is a property of electrically inexcitable mem- 
brane while initially excitatory for the associated 
electrically excitable spike generator can, secondarily, 
inactivate the latter and thereby block further conduc- 
tile or transmissional activity. 

This effect accounts for Wedensky inhibition, the 
failure of transmission produced by stimulating the 
presynaptic nerve at high frequency. Summated and 



sustained by this synaptic drive, the depolarizing 
p.s.p.'s at first evoke a few spikes which then cease to 
develop while the large p.s.p.'s continue to be pro- 
duced by the afferent stiinulation (fig. 9). Although 
Weden.sky inhibition is probably of little importance 
in physiological activity of organisms, the phenom- 
enon has long interested physiologists because the 
attempt to explain it in terms of electrical excitability 
has proved uncon\incing (cf 81, 141). The presence 
of electrically excitable and inexcitable electrogenesis 



i6o 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 




FIG. 12. Different effects on spikes and p.s.p.'s of cat motoneurons produced with different amounts 
of membrane polarization. Tlie membrane potential was changed by passing an appropriate cur- 
rent through the recording microclectrode. A Wheatstone bridge arrangement balanced out the 
artifacts caused by this current, but as a consequence absolute levels of the membrane potential 
could not be measured. Upper set (^A to G): Two traces are simultaneously recorded, the upper indi- 
cating the amount of current flow through the electrode, the lower showing the recorded potentials. 
.-1 to C, decreasing amounts of depolarizing current; D, no applied current; E to G, increasing 
amounts of hyperpolarizing current. The records are aligned so that the peaks of the spikes coincide 
(upper broken line). The parallel lower broken line passes through the point at which the spike 
begins. When the strong depolarizing current was applied in .4, it quickly evoked a direct spike. 
A subsequent orthodromic volley evoked a p.s.p. which reached the critical firing level but found 
the electrically excitable membrane still refractory. Hence, an orthodromically evoked spike was 
absent. At the end of this and subsequent records is a 50 mv calibrating pulse. B, C, the depolariza- 
tions from the applied current were smaller. They did not elicit a spike; but summing with the de- 
polarization of the p.s.p. evoked a spike earlier than the orthodromic volley alone did (D). Hyper- 
polarization of the membrane operated in the opposite direction, hindering the orthodromically 
evoked spike which appeared markedly late on the p.s.p. in F, and was absent in 6', although the 
p.s.p. in the hyperpolarized neuron was larger (compare the p.s.p.'s in A and G). A small deflection 
which follows the artifact of the stimulus to the nerve and precedes the p.s.p. by neaily i msec, is 
probably elcctrotonic pick-up of activity from the presynaptic impulses. Note that it is too small to 
evoke the spike. Lower set QA' to F'). In this experiment the spikes were elicited by antidromic in- 
vasion from the motor axons. A' to C, decreasing amounts of membrane depolarization; D\ no 
applied current; E' and F', currents applied so as to produce increasing membrane hyperpolariza- 
tion. The antidromic spike (Z)') shows an inflection which probably represents a response first in 
the axon hillock portion, succeeded by involvement of the rest of the cell. Depolarization of the 
cell body facilitates its invasion by the antidromic spike and minimizes the inflection on the rising 
phase. It is almost absent when the cell is strongly depolarized (.-l')- Hyperpolarization hinders the 
invasion of the cell body (F') and when it is strong (F') prevents the response of the soma. The 
smaller, early component is then seen in isolation as pick-up at the cell body of the response in the 
axon hillock and nerve fiber. Timing pulses at i msec, intervals are injected into the records. [From 
Frank & Fuortes (79).] 



in the same cell also permits blockade of spikes by 
synaptic depolarization induced by drugs that excite 
the synaptic membrane (fig. 13). This blockade is 
frequently useful clinically but it in often misnamed 
as'curarization" (cf. 96). Blockade by o'-tubocurarine 
and other similarly acting agents operates through an 



entirely different mechanism as will be described 
below. 

c) ELECTROCHEMICAL GRADATION AND REVERSAL OF 

POSTSYNAPTIC POTENTIALS. Although Synaptic trans- 
ducer action is not responsive to electrical stimuli, 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



l6l 




o 




D" 


0",; 


■^ 


— - — 


E" 


E"' 



•tz 




FIG. 13. Differential effects of depolarization on the spike 
and p.s.p. of the eel electroplaque. Column A to F, direct 
stimulation i columns A' to F', etc., weak, moderately strong, 
and very strong stimuli to a nerve. A to A'", the response of the 
normal cell. The resting potential is about 80 mv seen as the 
deflection of the active trace downward from the zero line 
(upper trace). The strong direct stimulus evoked a spike with 
very brief latency (.4). The weak neural volley caused a p.s.p. 
CA'"), the stronger also a spike (.-!" and .-!'") arising out of the 
p.s.p. The cell was treated with weak physostigmine (25 ^g per 
ml solution) for 78 min., and weak acetylcholine (i ixg per mg) 
for the last 58 min. of that period. These drugs had no effect 
on the potentials; 5 ftg per ml acetylcholine were then added. 
Depolarization developed, the spikes 36 min. later becoming 
smaller, but the p.s.p. was unaffected (S to /?'"). The diminish- 
ing electrically evoked response g min. later (C to C") became 
graded, as seen by its larger size in response to the strong neural 
volley. These effects progressed during the next ig min. (/) to 
Z)'") and 1 1 min. thereafter (£ to £""). The p.s.p. to the 
threshold neural volley decreased (£"). but was still evident 
later (F') when the electiically excitable membrane no longer 
responded to a much stronger direct stimulus (F). The p.s.p. 
to a maximal neural stimulation (f ") was still about as large 
as initially (^"')- This p.s.p. was capable of evoking a small 
graded response of the electrically excitable membi^ane, as seen 
by the delayed additional potential on the falling phase. [From 
Altamirano el al. (6).] 

the magnitudes of the p.s.p.'s and also their signs may 
be affected by changes in the membrane potential 
(52, 60, 97). These effects, however, are secondary 
and, indeed, are explicable only by the electrical 
inexcitability of postsynaptic electrogenic membrane. 
Suppose that a transducer action increases solely 
the permittivity for CI~. More of this ion being present 
in the external fluid, it tends to flow inward until the 
increased internal negativity tends to prevent further 
entry. Thus, the direction and amount of ionic flow 
is determined both by the chemical concentration 



gradient and by the electrical potential gradient, the 
coinbination being termed the electrochemical gradi- 
ent. For a given concentration gradient there is a 
corresponding potential gradient at which the flow 
of ions is balanced by the opposite force of the elec- 
trical charge. If the membrane resting potential is 
increased by some means, the electrogenesis caused by 
influx of Cl^ would reach the electrochemical poten- 
tial (Ecr) for that ion sooner. The hyperpolarizing 
p.s.p. would therefore appear to be smaller. If the 
membrane potential is made more negative than Eci~, 
Cl~ in the cell would be forced outward. The p.s.p. 
would then appear to reverse in sign, depolarizing the 
hyperpolarized membrane approximately to the level 
of Eci~. This effect is seen in figure 14I. 

The p.s.p. can likewise be affected by changing the 
Cl^ concentration either of the interior or of the 
exterior. For example, suppose that the external Cl~ 
is replaced by another anion which does not penetrate 
the membrane. During transducer action, Cl~ would 
move out from the cell since it is now more concen- 
trated in the interior. The electrogenesis of hyper- 
polarizing p.s.p.'s can thus be reversed to depolariza- 
tion. The effect of increasing internal Cl~ is seen in 
figure 14. Secondary electrochemical effects therefore 
can change the amplitude or sign of the p.s.p. 

In the case of the depolarizing p.s.p.'s, increase of 
resting membrane potential inay lead to increased 
electrical responses; decrease of the resting potential 
decreases and eventually reverses the sign of the de- 
polarizing p.s.p.'s. These various conditions for 
electrochemical grading and reversal of the p.s.p.'s 
are found experimentally (figs. 4, 11, 14). The grading 
and re\ersal of p.s.p.'s are strong evidence that the 
tran.sducer actions of synaptic membrane are not 
electrically excitable (97) since the physiological 
responses are not affected even by violent changes of 
the membrane potential, though the electrogenesis 
itself is modified. 

Cat motoneuron p.s.p.'s are electrochemically 
reversible (cf 60), but anomalies have been observed 
that are instructive. In theory, as outlined above, the 
apparent 'depolarization' of a reversed hyperpolariz- 
ing p.s.p. should only return the membrane potential 
to the saine level as does the hyperpolarization of the 
normal p.s.p. The 'depolarization' therefore should 
not reach the critical firing level for the spike, the 
membrane in theory still remaining at a hyperpolar- 
ized level, and the 'depolarizing' p.s.p. should not 
become excitatory. Frequently, however, this is not 
the case when the reversal is produced by changing 
the ionic concentration gradients of the motoneuron. 



j62 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 




IT 



CX " B I C 



FIG. 14. Reversals of hyperpolarizing p.s.p.'s. Intracellular 
recording from biceps-semitendinosus motoneuron of cat; hy- 
perpolarizing p.s.p.'s evoked by stimulating quadriceps nerve. 
I. A to G: The resting potential was —74 mv (Z)). Depolari- 
zation augmented the p.s.p. (.4 to C). Hyperpolarization at 
first diminished the p.s.p., the equilibrium potential for ionic 
movements without electrogenesis being at —82 mv (£). Fur- 
ther hyperpolarization reversed the sign of the p.s.p. (F, G). 
The Cl~ content of the motoneuron was then increased and 
K+ decreased (H to Z,). Immediately thereafter (J to Z.) the 
p.s.p. was 'depolarizing' at all but the least negative values 
'(//, /) of the membrane potential. M to Q: Recovery toward 
initial condition not yet complete 3 to 4 min. later. II. Reversal 
of the sign of the p.s.p. was produced by changing the ionic 
gradient of Cl~. Initial response 0-1) was altered in B and C by 
increeising intracellular Cl~ as a result of diffusion out of the 
tip of the microelectrode. Depolarization of the membrane to 
— 27 mv by an applied current restored the sign of the p.s.p. 
(D). The Cl~ gradient was then changed drastically. The re- 
versals of the p.s.p.'s produced soon thereafter (is to G) oc- 
curred without significant change of the resting potential and 
were sufficient to excite spikes, at first with brief latency (£), 
then progressively later (F and G). Each record is formed by 
superposition of many traces. In G it is seen that the depolari- 



The initial resting potential may then be altered little 
or not at all, as is also the case with microinjections of 
ions into squid giant axons (cf. 91, 105). The changed 
chemical gradient of the motoneuron then causes a 
reversal of hyperpolarizing p.s.p.'s into depolarization 
which develops at, or near, the initial resting poten- 
tial. The reversed 'inhibitory' p.s.p. now may elicit a 
spike (fig. 14II). 

In crustacean muscle fibers (68, 73) and stretch 
receptors (130) the equilibrium potential for the 
inhibitory p.s.p. is nearly identical with the resting 
potential. Stimulating the inhibitory axon therefore 
may elicit no p.s.p., or the latter may be small, and 
of either sign. Nevertheless, the membrane potential 
tends to be clamped at or near the resting potential, 
particularly if the activity of the inhibitory synaptic 
membrane increases markedly the permittivity of 
the membrane for the relevant ions (K+, Cl~ or 
both). Excitatory depolarization, elicited at the 
same time, by p.s.p.'s in muscle fibers or by mechano- 
sensory dendrites in stretch receptors, therefore tends 
to be depressed. When the inhibitory synapses of 
lobster muscle fibers are maximally activated by 
7-aminobutyric acid the membrane potential is 
increased by about 4 mv, but the membrane con- 
ductance is increased about 8-fold (Grundfest, 
Reuben & Rickles, in preparation; cf. 99). 

d) LATENCY OF posTSYN.-^PTic POTENTIALS. As men- 
tioned above, the onset of the explosive response of 
electrically excitable membrane depends upon the 
attainment of a critical level of depolarization. A 
strong electrical stimulus, causing rapid depolariza- 
tion to that level, therefore evokes a spike with vanish- 
ingly brief latency (fig. 6), this fact having been 
established by Bernstein in 1871 (19). In all cases 



zation occasionally fell below the critical firing level and con- 
tinued to decrease in the later records (// to L). III. The 
membrane generating hyperpolarizing p.s.p.'s maintains its 
pharmacological individuality, although the electrical response 
may be reversed and is then indistinguishable from that of a 
depolarizing p.s.p. Prior to taking this scries of records the 
hyperpolarizing p.s.p. evoked in the biceps-semitendinosus 
motoneuron by stimulating quadriceps afTercnts was reversed 
(by diflfusing Cl~ from the electrode into the cell). This response 
is shown at the beginning of each record (/). Following it is a 
depolarizing p.s.p. (£) evoked by stimulating afferents in the 
biceps-semitendinosus nerve. Strychnine salicylate (o. i mg per 
kg) was injected after record A and caused progressive diminu- 
tion of 7, but no change in £ during the next 4 lo-sec. intervals 
(fi to £). The reversed hyperpolarizing p.s.p. almost disap- 
peared after a second injection (f ). [From Eccles (60).] 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



163 



where appropriate data are available (of. 97) the 
neurally evoked response arises after an appreciable 
irreducible latency (fig. 6), or synaptic delay (44; 
cf. 140). 

Between the arrival of the presynaptic impulse and 
the onset of the p.s.p. of cat motoneurons there is a 
latency of about 0.3 to 0.4 msec. (59, p. 130). In the 
eel electroplaque the latency attains i to 2.5 msec. 
(4). This delay is not conducive to, nor consistent with, 
electrical excitation of synaptic membrane by the 
action current of the presynaptic impulse (97) as was 
pointed out by du Bois-Revmond (55) and Bernstein 
(20). 

Presumably, synaptic latency is compounded from 
the durations required: (T) for release of transmitter 
from the presynaptic terminals; («) for its transit 
across a synaptic space of about 100 A (52, 54, 152, 
153' '74' 553)' ^"^^ ("') fo'" development of the 
electrogenic reactions when the transmitter acts 
upon the postsynaptic membrane. The details of 
none of these components are as yet known. 

e) electrotonic effects of presynaptic impulse 
UPON postsynaptic region. Intracellular recording 
revealed (cf. 59, 60) that the presynaptic spike not 
only arrived too early, but also that its electrotonic 
efTect was too little to cause electrical excitation of the 
postsynaptic membrane. Indirect stimulation of the 
eel electroplaque (fig. 6C, D) excites the terminal fibers 
innervating the cell membrane. Their spikes must 
have occurred with vanishingly small latency upon 
strong stimulation (Z)). However, no trace of elec- 
trotonic effects in the electroplaque was found. The 
presynaptic impulses could not be observed even at 
high sensitivity of recording (fig. 3). In other prepara- 
tions small, brief as well as early electrotonic pick up 
of the presynaptic spikes is observed (cf. figs. 19, 
2 J A). The magnitudes, i or 2 mv, are insignificant for 
electrical excitation which requires critical depolari- 
zations of some 10 to 40 mv. 

Among the possibilities for accounting for the small- 
ne.ss of electrotonic effects across synapses are the 
following. 

/) Theresistanceof one or both cell membranes may 
be very high. In most types of synapses the presynap- 
tic terminals making contact with postsynaptic 
membrane are very small and this alone would de- 
crease the electrotonic effects. However, the contact 
between the pre- and postfibers in the .squid giant 
axon synapse are broad, yet the electrotonic post- 
junctional potential is small (fig. 19). Likewise, in the 
eel electroplaque where the innervation is diffused 



widely over the cell membrane electrotonic effects are 
small. 

2) The bulk of the synaptic current may be shunted 
by the subsynaptic space. 

3) If the nerve terminals were themselves elec- 
trically inexcitable neurosecretory regions the spike 
would not invade the nerve proximate to the synapse. 
The extrinsic current in the synaptic region would 
thus be already attenuated by electrotonic losses. 

f) CHEMICAL SENSITIVITY OF SYNAPTIC MEMBRANE. 

Many varieties of drugs exert effects upon synapses, 
but they either do not affect electrically excitable 
membrane or do so only when applied in high con- 
centrations and for long times (6, 96). The high 
sensitivity of synaptic membrane to chemicals is prob- 
ably also a con.sequence of its chemical excitability. 
Thus, many drugs cause synaptic electrogenesis, 
thereby mimicking the effects of the natural trans- 
mitter agents. The.se substances are known as ' de- 
polarizing drugs' but are more properly designated 
as 'synapse activators' (95, 96) for their action is 
merely that of excitants. The type of synaptic electro- 
genesis is determined by the nature of the synapse 
itself For example, acetylcholine and its mimetics 
cause depolarization when applied to muscle end- 
plates or sympathetic ganglia, but when applied to 
the cardiac pacemaker synapses which are hyper- 
polarized by vagal stimuli the drugs also cause hyper- 
polarization (49, 1 20). A second group of substances, 
the' synapse inactivators', hinder or prevent excitation 
of the membrane bv the activator drugs. These are 
also called ' nondepolarizing competitive inhibitors' 

(155)- 

Both types of substances may cause block of trans- 
mission. Depolarizing excitatory p.s.p.'s are dimin- 
ished in amplitude or prevented by the inactivating 
drugs. The decrease of the p.s.p. below the critical 
level for discharging spikes is the mechanism of the 
synaptic blockading action of these drugs. Curare or 
</-tubocurarine act in this way (fig. 15). A general 
feature of blockade by inactivating drugs is that the 
electrically excitaijle membrane is affected little or 
not at all. Thus, the postjunctional cell can remain 
directly excitable. 

Synapse-activating drugs induce transmissional 
blockade by an entirely different mechanism which is 
referrable to the fundamentally different excitabilities 
of electrogenic membrane. Acting on the synaptic 
membrane, the drugs evoke depolarization of the 
excitatory synapses. This electrogenesis, sustained in 



164 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 




FIG. 15. Effect of nondepolarizing synaptic blocliing agents 
on the responses of the eel electroplaque. Direct stimulation of 
the cell is represented in A to F, neural excitation in A' to C. 
The initial responses to both stimuli are shown in .4 and A'. At 
3 min. after substituting a bathing solution containing 5 mg 
per ml o'-tubocurarinc, the directly elicited spike was unchanged 
(B), but synaptic excitation was less effective, the spike arising 
later on the smaller p.s.p. (£')■ A* 5 ""in. (C, C) the directly 
elicited response was still unaffected, but the p.s.p. had de- 
creased so much (,C") that it was seen only with repetitive 
stimulation at 50 per sec, and produced a single small 'spike,' 
after which it could no longer affect the electrically excitable 
membrane. The latter, however, remained fully responsise to a 
direct stimulus 41 min. later (/)), but eventually this responsive- 
ness decreased (96 min. later, E; and 1 1 o min. after this, F). 
The resting potential of the cell was unchanged. Calibration 
100 mv and msec. [From Altamirano et al. (6).] 



the presence of the chemical stunulant, leads to 
inactivation of the spike-generating membrane as 
described above. The entire cell may then become 
inexcitable by direct stimuli (fig. 13). In the case of 
skeletal muscle fibers, the inactivating depolarization 
is confined to the regions of the endplates and neuro- 
muscular transmi.ssion is blocked Ijecause these 
regions do not generate spikes. Neuromuscular jjlock- 
ade evoked by 'depolarizing' synap.se activating drugs, 
and blockade also at neuronal synapses, are usually 
preceded by a brief period of hyperactivity. The dis- 
organized contraction of muscles, frequently but 
incorrectly termed 'fasciculation', is due to the initial 
excitatory effect of the synaptic depolarization, the 
individual muscle fibers responding to this stimulus 
before their spikes are inactivated. Blockade by the 
truly curarizing drugs, the inacti\ators of synaptic 
activity, is not preceded l)y the excitatory eflfects. 



Postsynaptic Potential': as \onpropagated 
' Standing' Potentials 

The local circuit current of activity, in combination 
with electrical excitability, makes possible the con- 
ductile property of electrically excitable, regenera- 
tively responsive membrane (fig. 8). The all-or-none 
character of the spike then leads to decreinentless 
propagation. A consequence of electrical inexcitaljil- 
itv is that the p.s.p.'s do not set off activity in other 
portions of synaptic membrane. The electrogenesis is 
therefore localized and does not propagate except 
electrotonically as mentioned earlier (fig. 2). This 
'standing' nature of p.s.p.'s has important physiologi- 
cal consequences that will be discus.sed later. It also 
introduces a technical complication in the interpre- 
tation of potentials recorded from volume conductors. 
The rules that apply to potentials generated by a 
travelling impulse (cf. 140, 141) need not hold, 
particularly since hyperpolarizing as well as depolar- 
izing p.s.p.'s of the ' standing' variety can be produced 
at various sites (cf. 161-167).' It is of more than his- 
torical interest to note that Sherrington and his 
colleagues sue:gested that the central excitatory state 
(c.e.s.) "is a specialized manifestation of local exci- 
tatory state." (44, p. 43). In the present da\- contexts, 
the central excitatory state may be identified in large 
measure with occurrence of depolarizing p.s.p.'s, 
and the central inhibitory state with of hyperpolariz- 
ing p.s.p.'s. However, phenomena such as desensitiza- 
tion (p. 157) may obscure or eliminate this parallelism 
between potentials and excitability. Thus, as appears 
in figure 11, the depolarized but also desensitized 
endplate may not respond to a stimulus. Such a condi- 
tion might lead to blockade of transmission although 
the background is one of depolarization. Desensitiza- 
tion of hyperpolarizing synapses has not yet been 
described, l)ut its occurrence is not unlikely. If it 
exists, it could provide cases of lifting of inhibitory 
blockade in the face of a background of hyperpolariza- 
tion. It will be shown later that the responsiveness of 
electrically excitable membrane (its local excitatory 
state) can change without a parallel change of the 
membrane potential, although the excitability of this 
meinbrane is also a reflection of the action of tjraded 
local responses. 

' An extreme example of localized activity which is therefore 
highly instructive has been reported in the cat cortex (150, fig. 
19). Within a range of 20 /i in the depth of the cerebral cortex 
the pattern and degree of electrical acti%ity undergoes great 
modifications. 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



165 



Interaction of Graded Responses 

Generated and propagated in electrically inexcit- 
able membrane, p.s.p.'s can spread only by electro- 
tonus (fig. 2), passively, without evokina; new activity 
and with considerable decrement. As weak depolariza- 
tions, p.s.p.'s acting upon adjacent, electrically excit- 
able membrane may evoke graded local responses 
(figs. 3, 7, 13). The latter are also decrementally 
propagated, but the decrement may be smaller than 
in the case of p.s.p.'s. The depolarizing activity of a 
graded local response at one site may, in turn, give 
rise to some degree of active response at other sites. 
Thus, depending upon the local excitability of the 
membrane and the amount of initial local response, 
this graded depolarization may spread only passively, 
or it may propagate with various degrees of active 
contribution. The ultimate extent of the latter is that 
which evokes a spike. This explosive process domi- 
nates subsequent events since the magnitude of its 
electrical activity usually far exceeds the require- 
ments for continued local circuit electrical excitation. 
In other words, when the spike generator has a high 
safety factor, decrementlcss propagation is the rule. 

The nature of graded local responses of electrically 
excitable membrane will be discussed below (p. 167) 
in conjunction with the mechanisms of gradation of 
p.s.p.'s. Here, it is desired to stress that the two 
graded responses provide a pathway for summative 
gradation as a transition to the all-or-none spike 
(fig- 3)- 



EVENTS IN SYNAPTIC TRANSMISSIO.V 

Functional Interrelations Within Single Cell 

A generalized schema of the activities within a 
single unit in a transmission chain is shown in figure 
16. The input of the cell, the synaptic surface in the 
present context, but which may also be the receptor 
surface of a sensory cell (cf. 94, 96, 97), is activated 
by a specific chemical stimulus and develops an 
electrical response. Only the depolarizing variety, 
excitatory for the conductile mechanism, need be 
considered now. The p.s.p. may ije brief or long and 
may give rise to a single spike or to a train of impulses. 
This conductile activity, arriving at the terminus of 
the cell, causes secretory activity which releases a 
transmitter agent that can excite another unit of the 
transinission chain or an effector. 



INPUT , CONDUCTILE | OUTPUT 




GENERATOR 
ACTIVITY 



CONDUCTILE 
ACTIVITY 



TERMINAL 
ELECTROGENESIS 



FIG. 16. Diagrammatic representation of functional com- 
ponents and electrical responses of a receptor cell or neuron. 
The electrically inexcitable input produces electrogenesis 
graded in proportion to its specific stimulus and sustained as 
long as the latter is applied. The possibility of hypcrpolarizing 
electrogenesis is shown but is not further considered. The de- 
polarization at the input, operating upon the conductile elec- 
trically excitable component, can evoke spikes in the latter 
coded in number and frequency in proportion to the depolari- 
zation. These signals, propagated to the output, there command 
secretory activity, roughly proportional to the information en- 
coded in their message and sustained as long as the message 
demands. The transmitter released at the output can initiate a 
synaptic transfer by operating upon the depolarizing input of 
another cell. The possibility of a special output electrogenesis 
is indicated but is not further considered. The lower electrical 
portion of this diagram may be compared with records from a 
sense organ (fig. 10). [From Grundfest (97).] 



Evolution of Electrogenic Membrane 

The occurrence of receptor-effector cells in primi- 
tive metazoa suggested to Parker (154) that the nerv- 
ous systein ev'olved by parcellation of the two func- 
tions among separate receptor and effector cells with 
the interposition of a conductile element extending 
from the receptor cell. Later in evolution, correlational 
neuronal cells were presumed to have arisen. This 
evolutionary schema may also be applied to the 
individual cells, neurons and muscle fibers as well as 
receptors (103). The receptor portion of the priinitive 
unit was probably sensitive to specific stimuli and this 
characteristic is retained at the electrically inexcitable 
input of the present nerve cell, mu.scle fiber, gland or 
receptor (fig. 16). The ouptut likewise inay be con- 
sidered as representing the primitive effector, frankly 
so in the contractile muscle fibers or in glands. The 
terminals of the neurons likewise probably embody 
the secretory capacity of primitive units adapted to a 
new function, transmission at close contact. Other 
neurosecretory cells of more general function are also 



1 66 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 1 



common (179) and the electrically inexcitable secre- 
tory cells of the adrenal medulla are regarded as 
second order autonomic neurons (cf. i 77). 

The conductile portion of the neuron, generating 
all-or-none spikes and therefore capable of decre- 
mentless propagation, requires electrical excitability 
for this function. It is probably a later evolutionary 
development (21) brought about in the course of 
extension of the cells in the metazoa and of their 
participation in complexly organized activity. That 
the conductile activity represents a new evolutionary 
step, mediated by a structure interposed between the 
primitive input and output sections, is also suggested 
by the absence of conductile electrogenesis in gland 
cells and by their electrical inexcitability (96, 97). 
The occurrence of muscle fibers which are also not 
electrically excitable and which generate no spikes 
(4, 34, 35, 97) reinforces this view. Classifying distinc- 
tions with respect to excitability and the types of 
responses of electrogenic membranes are by no means 
exhaustive of the different varieties. Pharmacological 
differences of various kinds specify an even greater 
diversity amongst excitable, electrogenic membranes. 
These differences are not to be seen by anatomical 
methods, nor indeed, by electrophysiological means 
alone, since pharmacologically distinct varieties of 
membrane can all generate similar types of electrical 
responses (fig. 14III). 

Transmitter Actions 

The varieties of transmitters will be treated below; 
the present discussion will be confined to the general 
electrophysiological aspects. From this point of view, 
the precise chemical natures of the substances are of 
little moment, the important feature being that they 
all activate synaptic electrogenesis. It is unlikely that 
the sign of the p.s.p. is affected by the excitant agent. 
Thus, as noted above, acetylcholine is a 'depolarizing' 
substance for excitatory p.s.p.'s but activating inhibi- 
tory synapses, as in the pacemaker of the heart it is a 
" hyperpolarizing' agent. The characteristics of the 
transmitters will, however, determine to .some extent 
the character of the p.s.p. a) For example, if the 
transmitter is a large complex molecule, it is unlikely 
that it would be available in large concentrations at 
the terminals of the presynaptic element. The amount 
of total excitant might therefore be limited in propor- 
tion to the quantity secreted during a single activity. 
Thus, a single afferent volley might cause a number 
of p.s.p.'s, but repetitive activity might rapidly ex- 
haust the available transmitter, b) Molecular dimen- 



sions and configurations might also determine the 
rapidity of diffusion of the transmitter from its site of 
release to its site of action. The distances involved, 

o 

although probably only about 100 A are significant 
in terms of molecules, c) The kinetics of interaction 
between the transmitter and the postsynaptic electro- 
genic surface may also be in part determined by the 
transmitter itself. For example, it is conceivable that 
two different agents might act on similar synaptic 
sites with different kinetics, giving rise to differences 
in the p.s.p.'s evoked by each. Studies in kinetics of 
these interactions are only now beginning (cf 53, 127) 
and the nature of interaction is as yet unknown. 
Analogy with other processes is usually invoked and 
two models which are at present fashionable, actisa- 
tion processes of enzyme reactions and antigen- 
antibody combinations, are not necessarily mutually 
exclusive. The transmitter agent is presumed to com- 
bine with some ' receptor' sites of the synaptic mem- 
brane (cf 2, 9, 14). (f) The chemical properties of the 
transmitter may also determine the characteristics of 
the p.s.p. Thus, a labile agent such as acetylcholine 
may be rapidly destroyed, and it might give rise to 
shorter p.s.p.'s than would a more stable excitant of 
the same synaptic site (cf. 53). Likewi.se, the degree 
of chemical binding between the transmitter and the 
' receptor' or the stability of the complex may play 
similar roles in determining the duration of the p.s.p., 
or in its 'competitive' behavior toward an inactivating 
synaptic drug. /) Although a transmitter agent may 
activate a given type of receptor it may also be an 
inactivator of other types. Thus, the transmitter at 
inhibitory synapses of some invertebrate muscle fibers 
is thought to be an inactivator of the excitatory syn- 
apses (68, 73). g~) A given synaptic complex might be 
composed of several \arieties of receptors, although 
all generating the same kind of p.s.p. Yet, one trans- 
mitter might inactivate some of the receptors while 
another transmitter did not, and the p.s.p.'s would 
vary accordingly. 

Two of the factors, the transit time of the trans- 
mitter across the synaptic gap (6 in the preceding) 
and an induction period (c above), probably deter- 
mine the synaptic latency as noted earlier. Together 
these two processes may last several milliseconds. 

Genesis nf Postsynaptic Potentials 

Important information on this matter derives from 
the occurrence of spontaneous 'miniature' p.s.p.'s at 
muscle endplates. Probably this activity is generated 
bv random releases of transmitter from presynaptic 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



167 



sites (52). The miniature p.s.p.'s are probably quanta! 
in the sense that each is composed of a minimum 
electrical change generated by a 'packet' of trans- 
mitter agent. The random release of packets from 
presynaptic terminals at different synaptic sites and 
the electrical inexcitability of the postsynaptic mem- 
brane combine to cause local miniature p.s.p.'s gener- 
ated now at one site, now at another (51). 

Depolarization of the presynaptic nerve terminals 
augments the frequency of miniature e.p.p.'s in frog 
muscle fibers (52). Similar data (137) on rat dia- 
phragm muscle are even more decisive (fig. 1 7). 
Depolarizing electrotonus applied to the phrenic 
nerve increases the rate of the miniature activity very 
markedly, while hyperpolarizing the nerve terminals 
decreases the activity. E.xcess magnesium, which de- 
presses the release of transmitter agents (cf. 52), 
depresses or eliminates the effects of the electrotonic 
currents. 

The action of magnesium indicates that the effects 
produced by the electrotonic potentials are exerted 
through the medium of the nerve terminals and are 



100 

50 

c 
c 
o 

'- 5 



.»- Anodic Cathod'C -». 



FIG. 17. Effects upon the frequency of miniature e.p.p.'s in 
rat diaphragm muscle fibers of electrotonus appHed to the 
phrenic nerve. Abscissae show the intensity of applied electro- 
tonic current in relative units; ordinates, the frequency of 
miniature e.p.p.'s scaled logarithmically. Arrows point to 
frequencies of the latter observed when no electrotonic currents 
were applied. 'Cathodic' current is depolarizing for the nerve 
terminals, anodic' is hyperpolarizing. A: The eflfects of the 
change in potential were essentially symmetrical on the loga- 
rithmic scale, increa.sed frequency of miniature e.p.p.'s with 
cathodic and decreased frequency with anodic current. This 
was the most frequently encountered result. B: open circles, 
terminal depolarization was much more efTective than hyper- 
polarization in changing the frequency in this experiment; 
Jilled circles, the same inuscle was exposed to 12 mmole mag- 
nesium (normal concentration is i mmole). The frequency of 
miniature e.p.p.'s became essentially independent of the mem- 
brane potential of the nerve fibers. [From Liley (137).] 



genuinely synaptic in nature. Other tests also lead to 
this conclusion, a) The electrotonic effects on the 
miniature e.p.p.'s are absent in muscle fibers where 
the nerve supply is cut close to the muscle and thereby 
made inaccessible to the electrotonic currents. This 
rules out the possibility that the current flow in the 
muscle fibers themselves caused the changed rate of 
miniature e.p.p.'s. b') The effect of the electrotonus 
was absent in endplates that were more than a few 
millimeters from the site of applying the stimulus to 
the nerve. Since the decay of electrotonically spread 
potentials must be rapid in the terminal nerve fibers, 
this result indicates that the change in rate is initiated 
by effects in the presynaptic terminals. These experi- 
ments show that when the depolarization produced 
by a nerve impulse arrives at or near the presynaptic 
terminals, their secretory activity can be initiated or 
augmented. A mechanism coupling the presynaptic 
impulse and transmission is thus provided. 

Some additional conclusions may be deduced from 
data on miniature e.p.p.'s. These activities increase in 
frequency approximately lo-fold for 15 mv depolari- 
zation (137). Therefore a spike, though lasting only a 
brief time, could mobilize the rapid release of a 
considerable number of transmitter packets since 100 
mv depolarization might increase the rate of ' spon- 
taneous' releases some 10' to 10* times. The number 
of packets involved in an e.p.p. during neuromuscular 
transmission is probably about 10- to 10^ times the 
'quantal' units that cause the miniature e.p.p.'s (52). 

Increase in the rate of release or .secretion of the 
transmitter at the presynaptic terminals is obviously 
an electrically activated event. However, the response 
at the effector terminals probably differs Ijasically 
from the processes that generate the spike of the con- 
ductile membrane. The data of figure 1 7 were ob- 
tained with prolonged applications of electrotonic 
currents. The sustained increase of miniature e.p.p.'s 
during sustained depolarization therefore indicates 
that the processes leading to release of transmitter 
packets are not subject to inactivation as is the sodium 
conductance of the spike generator. 

Gradation of Postsynaptic Potentials 

Probably the miniature p.s.p.'s are small only be- 
cause the area involved in their electrogenic activity 
is small in comparison with the total area over which 
the emf is electrotonically distributed. .Suppose that 
we could measure the change in potential occurring 
at a single isolated site which valves sodium ions. In 
the resting state the emf across that site will be the 



1.68 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



resting potential, approximately the value of the po- 
tassium potential, EsCgi, 112, 113). When the valve 
■ opens' the emf must suddenly change from the resting 
value towards that of the sodium potential, £>,-,,, a 
change to internal positivity (113). The hypothetical 
'valve', however, is located in a physical structure, 
the membrane, with finite resistance and capacity 
and with both its surfaces bathed in saline media. 
The step-like emf of the generator ' valve' must there- 
fore distrilauie itself electrotonically o\er an area hav- 
ing definite electrical properties, becoming a potential 
change reduced in magnitude and distorted in form 
(fig. 2). The simultaneous activity of a number of 
"valves' would lead to an increased potential, thus 
permitting gradation of the response from the mini- 
mal observable to the full value of the electrochemical 
potential. Since several species of ions are invoked, 
the maximum p.s.p. strikes a i)alance ijetween the 
different electrochemical potentials (cf. 52, 60). 

Aferlianisms of Graded Responsiveness 

The most detailed data are available on graded 
responses of electrically excitable membrane and, al- 
though the theory of their production is still rudi- 
mentary, the same general process will probably be 
found to apply also to the graded responses of synaptic 
and sensory membrane (94-96). Graded local re- 
sponse is usually considered to be merely a stage in 
the events leading to the regenerative explosive activ- 
ity which results in the spike (113). This view has 
been invalidated by the finding (4, 6, 92) that under 
various conditions all-or-none responsiveness can be 
converted to a fully graded one. Only graded re- 
sponses occur in dually-responsive insect muscle 
fibers (37, 38) and probably in other electrically 
excitable membranes as well (97)- The activity may 
vary from the minimal observable to a maximal 
response closely approximating the spike in amplitude 
and form (fig. 18; cf. fig. 21). The degree of graded 
responsiveness is not controlled by the membrane 
potential as it is considered to be in current theory 

Figure 18 also illustrates how an altered local 
excitatory state need not be caused by, nor reflected 
in, a changed membrane potential. Whether un- 
treated or poisoned with a drug, the single cell showed 
subliminally enhanced excitability which w-as evi- 
denced during an interval at least 0.2 sec. after each 
subthreshold stimulus. The cumulative growth of this 
"excited' state in the untreated cell led to an explosive 
manifestation, the spike. After the cell was poisoned 



the overt manifestation look the form of a progres- 
sively larger graded response, and this response 
approached the spike in amplitude. 

A first approximation for revising theoretical con- 
cepts (94, 95) con'iiders that the excitalile memiirane 
is composed of unit areas. Each has a population of 
electrogenic units (transducers, valves, etc.) which 
differ amongst themselves in the threshold for their 
excitation. In the explosively responsive population 
the thresholds for exciting the electrogenic elements 
of a given unit area are probably closely similar. 
Dispersion of that population distribution could re- 
sult in conver-^ion of all-or-none responsi\eness to the 
graded type. 

Transfer of Activity From Postsynaptic Potentials to 
Electrically Excitable .Membrane 

In the case of the skeletal muscle endplate or the 
squid giant fiber synapse a relatively well-defined 
' patch' of electrically inexcitable synaptic membrane 
is surrounded by electrically excitable structure. In 
both cells, the p.s.p. is simple, only of the depolarizing 
variety, and initiated by impulses in a single pre- 
synaptic fiber (fig. 19). The p.s.p. then tends to be of 
a fixed amplitude and in these two systems usually 
causes sufficient depolarization of the contiguous 
electrically excitable membrane to generate a spike 
in the latter. Essentially, transmission then is one-to- 
one, each impulse of the prefiber generating a post- 
junctional spike. 

Under various conditions, for example upon poison- 
ing an endplate with rf-tubocurarine, the p.s.p. de- 
creases in amplitude and, when the depolarization 
falls below the critical firing level, no spike is gener- 
ated (fig. 15). The transmission block may be over- 
come by a rapidly repetitive volley of neural stimuli 
which successively generate new p.s.p.'s or a local 
excitatory state before the previous ha\e disappeared. 
The consequent augmentation of depolarizing elec- 
trogenesis may attain the critical level and transmis- 
sion again occurs. This general phenomenon of 
increased effectiveness of repetitive stimuli is known 
as facilitation. The normally occurring p.s.p. produced 
by a given afferent neural stimulus may not be suf- 
ficiently large to evoke a spike. Repetitive stimuli in 
this case can summate the depolarizing p.s.p.'s and 
facilitation is then also manifested, the summed de- 
polarization initiating a spike. 

In the context of the electrophysiological mecha- 
nism, a facilitated overt respon.se (e.g. of a muscle) 
may be produced by two fundamentally different 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



169 




A. 



JV. 



A. 



B 



-V 



/v. 



-V 



.^ 



JV 



.^. 



.^. 




.^^ 



-^ 



.^ 



.^ 



-^ 



8' 



.^. 




FIG. 18. All-oi-none and graded responsiveness in an eel 
clectroplaque. Two traces are recorded simultaneously, re- 
peated at the rate of 5 per sec. The upper longer trace of each 
set is the zero base line for an internal microelectrode. It also 
carries the monitoring signal of a stimulus applied to the cell 
and shows that the stimulus strength remained constant in 
each of the two series. The lower trace of each set is that of the 
potential recorded with the microelectrode. The distances be- 
tween the two represent the resting potential, about 70 mv. 
The weak stimulus in A, before the cell was treated with drug, 
at first produced only a subthreshold electrotonic depolariza- 
tion. The seventh repetition of the stimulus is followed by a 
spike. The shorter latency at which successive spikes then de- 
velop indicates continued growth of excitability and its per- 
sistence through the 200 msec, intervals between stimuli. The 
resting potential remained unchanged. B and B' . The sequence 
of growth in response in the cell after 84 min. exposure to 500 
Mg per ml of physostigmine. The resting potential was not 
affected by the drug, which eliminated synaptic excitability 
and converted the all-or-none response of the electrically 
excitable membrane component to graded responsiveness. The 
testing stimulus was slightly stronger than before applying the 
drug, and the first trace seen (upper set of S) evoked a distinct. 



synaptic processes. The one descrif)ed just above is 
summation where each successive p.s.p. is no larger 
(cf. fig. 27.4), and may indeed be smaller, than its 
predecessor. The excitatory action leading to the 
overt effect would be the increased total depolariza- 
tion produced b\- the summed effects of the repeated 
p.s.p.'s. The overt effect would appear as a facilita- 
tion because of the profound functional difference 
between the local processes at the motoneuronal or 
neuromuscular synaptic junction and their production 
of an explosive propagated spike which triggers the 
contractile mechanism. 

Essentially the same overt result, but an activity 
involving more complex synaptic processes, would 
(jccur if the successive p.s.p.'s augmented as a result 
of the repetitive stimulation. This synaptic facilitation 
will be discussed further in relation to heterosynaptic 
and homosynaptic excitatory phenomena (p. 184). It 
would seem to involve augmented responsiveness of 
the synaptic membrane to the transmitter agent, the 
converse to the decreased responsiveness in desensiti- 
zation. As noted in that connection, defacilitation 
probably is ascribable to desensitization. Both facili- 
tation and defacilitation, however, may be only 
apparent effects on the synaptic membrane, their real 
cause residing elsewhere. For example, facilitation 
could result from successively larger quantities of 
transmitter released from the presynaptic terminals. 
The converse, progressive exhaustion of the trans- 
iTiitter and decrease of the amount emitted at each 
impulse, would lead to defacilitation. 

As is also the case with other electrical stimuli, the 
depolarizing p.s.p. first evokes a graded local response 
of the electrically excitable membrane (4) and the 
two depolarizations then sum to cause the explosive 
response of the spike (figs. 3, 7). The addition of 
hyperpolarizing p.s.p. to the depolarizing diminishes 
the magnitude of the latter and its excitatory effect. 
If the depolarizing p.s.p. then falls below the critical 
level, a spike is no longer elicited and the effect of 
hyperpolarizing p.s.p.'s is therefore inhibitory. It 
should be noted that inhibition may occur even 
though considerable depolarization is still generated. 
In other words, the countervailing inhibitory p.s.p. 



though small, graded response. During the course of repetitive 
stimulation at 5 per sec. the response grew, at first gradually 
and then more rapidly, indicating that the rise of excitability 
is non-linear. The series illustrated ends before the response 
could grow to an amplitude as large as that of the spike, but 
in other experiments this was observed. [From .Mtamirano el 
al. (6).l 



1 70 



HANDBOOK OF PHVSIOLCKJY 



NEUROPHYSIOLOGV 




I I I I I I I 

msec 

FIG. 19. Synaptic transfer in squid giant axons. The incom- 
ing presynaptic spike elicits only a small membrane potential 
change in the postsynaptic cell. The p.s.p. arises after a brief 
latency and, if it attains the critical firing level, elicits a spike. 
[From Bullock & Hagiwara (32).] 



need not be as large as is the excitatory one. It must 
only be large enough to decrease the depolarizing 
p.s.p. below the critical firing level for the spike, but 
it can then produce dramatic effects since the absence 
of conductile activity eliminates further transfer to 
other cells and results in the disappearance of distant 
actions within the organism. 



Synaptic Delay 

Synaptic latency, which was discussed above, in- 
volves only the activity of the presynaptic terminals 
and the response of electrically inexcitable synaptic 
membrane. Synaptic delay includes not only the 
latency but also the utilization time of electrical 
excitability. This last involves the duration of the rise 
of the depolarizing p.s.p. and of whatever further 
depolarization this may develop in its excitatory ac- 
tion on electrically excitable sites, and the consequent 
time that is required for the p.s.p. (and the local 
response) to reach the critical level for evoking a 
spike. The rise time of the p.s.p. for this level may be 
brief, about o. i to 0.3 msec. (figs. 6, 12), but can be 
much longer (figs. 7, 9), particularly if the depolariz- 
ing p.s.p. is liminal for discharge of the spike. Tem- 
poral summation or facilitation, in which repetitively 
evoked depolarization becomes larger, may then de- 
crease the utilization time and thereby shorten the 
synaptic delay (cf. 140). The shortening might also 
occur because of decreased synaptic latency or 
heightened synaptic excitability, effects which are 
discussed in the next section of this chapter. 

The existence of synaptic delay has been a.scribed 
chiefly to slowed conduction of the afferent impulse 
in the fine terminals of the presynaptic fibers (cf. 



57, 140). That explanation is no longer tenable. 
Strong electrical stimuli directly applied to the inner- 
vated surface of the eel electroplaque, and therefore 
to the nerve terminals, nevertheless cause a neurally 
evoked response always after a considerable synaptic 
latency (fig. 6). Further evidence may be derived 
from figure ig and other data of similar nature which 
show that the presynaptic spike arrives at the synap- 
tic surface somewhat before the p.s.p. is elicited. Thus, 
.synaptic latency and the utilization time involved 
in the rise of the p.s.p. to the critical firing level are 
probably the major factors in synaptic delay. 

Sujifrjuisition of Pustsyriafitic Potentials and Spikes 

The electrically inexcitable generators of p.s.p. 's act 
independently of and in parallel with the electrically 
excitable membrane that produces the spike (4, 
48, 71). Thus, a p.s.p. can be evoked during the 
spike, when a second response of the electrically 
excitable membrane is impossible due to its absolute 
refractoriness (figs. 6, 7). However, the combined 
response depends upon the prevailing electrochem- 
ical conditions of the cell. The p.s.p. may subtract 
from as well as add to the spike, the former occiu'ring 
when the spike itself carries the membrane potential 
into the region at which the p.s.p. reverses as de- 
scribed above (48, 136; cf 97). The conclusion that 
the spike under certain conditions wipes out the 
p.s.p. (cf 60, p. 30 ff) may therefore require revision. 
A complicating factor that ma)' explain these find- 
ings of Eccles and his colleagues is the distortion 
produced in the spike when the latter is elicited in a 
depolarized electrically excitable membrane (cf 
95). An "undershoot' of apparently hyperpolarizing 
phase then terminates the spike, even though it is 
absent in the response evoked at the normal resting 
potential of the membrane (fig. 10; cf 60, fig. 16). 
The distortion is probably due (95) to excess of po- 
tassium conductance over the sodium conductance 
as in .squid giant axons (113). This excess would be 
caused h\ increased sodium inactivation produced 
by the depolarization. 

The foregoing remarks indicate that electrical and 
physiological conditions of the soma membrane affect 
the recording of celhdar potentials. The soma, how- 
ever, is only one part of the cell, although it is the 
one most easily accessible to microelectrodcs. Even 
in neurons without dendrites, as is the case in tissue- 
cultured dorsal root ganglion cells, the intracellularly 
recorded response to stimuli may take on complex 
forms (42). This indicates that activits in and the 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



171 



properties of the axon contribute to the potential re- 
corded from the soma. 

The nature and degree of excitability may be dif- 
ferent in various parts of the soma and dendrites. 
Thus the soma may be electrically inexcitable (ry, 
33, 80, 186, 189, 190). The depolarizing p.s.p.'s 
or generator potentials cv'oked at the soma excite 
spikes at electrically excitable regions some distance 
from the cell body. The superficial portions of apical 
dendrites in the cat cortex are not electrically excit- 
able (loy, 165), As mentioned earlier, the receptor 
portions of various sensory cells are electrically in- 
excitable and for this reason are capable of develop- 
ing a sustained generator potential.^ 

Recent evidence (7, 17, 43, 82) also indicates that 
different portions of electrically excitable mem- 
branes of the cell body may have different thresh- 
olds. The ■ initial segment' of the motoneuron (cf. 
60) in the cat (82) and toad (7) responds first to an 
electrical stimulus and gives rise to the early part of 
the antidromic spike (fig. I2^'-F')- The spike of the 
rest of the cell body (if the latter is electrically ex- 
citable) occurs slightly later, the delay giving rise 
to a slight break in the recorded response. 

In addition to these apparent inhomogeneities in 
the excitability of different parts of the soma and 
dendrites, slowed conductile spread, separate loci of 
origin for spike and p.s.p.'s and different loci for 
depolarizing and hyperpolarizing p.s.p.'s are all 
factors that may contribute to variations in the 
recorded response of the cell. Many variations can 
be theoretically deduced, but their analysis is beyond 
the present scope. 

'Retinal receptors in lisli (184) provide an interesting new 
example (102). Their electrical response is probably generated 
in cells other than the primary visual cells (cones). The re- 
sponse is a sustained clectrogenesis. In some cells it is only 
hyperpolarizing, in others depolarization is also developed, 
depending upon the wavelength of the stimulating light. The 
amplitudes of the responses are graded, not only with the in- 
tensity of the light stimulus but also with its spectral composi- 
tion. These characteristics of electrically inexcitable activity are 
produced apparently in the absence of spikes, but the electro- 
genesis, both hyperpolarizing and depolarizing, affects spike 
production in other conductile elements. It has been suggested 
(102) that these electrogenic cells (probably horizontal or bi- 
polar cells or both) are excited by transmitter agents released 
by photochemically activated cones. The clectrogenesis, in 
which an electrically excitable component is lacking, is in turn 
associated with secretory activity as in electrically inexcitable 
gland cells. The secretory products acting upon the retinal 
ganglion cells evoke neuronal activity of the latter, probably 
including excitatory and inhibitory p.s.p.'s which lead to 
patterns of spike activity seen in the optic nerve fibers. 



GENERAL AND COMPARATIVE PHYSIOLOGY OF SYNAPSES 

Forms and Magnitudes of Postsynaptic Potentials 

Viewed as the nonregenerative responses of elec- 
trically inexcitable membrane, the forms and mag- 
nitudes of the p.s.p.'s may be expected to have 
rather simple relations to their excitants. The availa- 
ble experimental data are as yet rather scanty, but 
they do permit some general conclusions (cf. 60, 97). 

As a first approximation, the degree of synaptic 
transducer action reflected in the rate and amount 
of clectrogenesis may be considered to be roughly 
proportional to the quantity of excitant. A brief jet 
of labile transmitter or activating drug causes a 
Ijrief response while the continued availability of 
the excitant causes a sustained clectrogenesis. The 
duration of the p.s.p. in the first case will be deter- 
mined by the time course of the transducer action 
initiated by the excitant (cf. also 53, 127, and papers 
cited there). However, the responses will be dis- 
torted by the electrical circuit properties of the 
membranes. Thus, the rising and falling phases of 
the p.s.p. may reflect this distortion which produces 
a slowing such as occurs in electrotonic propagation 
(fig. 2). The rise of the p.s.p. should be slowed less 
than its fall since the former occurs when the mem- 
brane resistance and time constant are relatively 
low. This is the case experimentally as numerous 
figures in this chapter indicate. The falling phase 
probably bears some relation to the time constant 
of the membrane (cf. 60), lasting longer when the 
time constant is larger, like the ballistic response of a 
slow galvanometer to a brief current. The relation, 
however, does not appear to be a simple one (95, 
97), and the duration of the p.s.p. probably reflects 
importanth intrinsic time courses of transducer ac- 
tions. 

The duration of the p.s.p. caused by a single 
neural volley differs considerably in the various types 
of cells. The p.s.p.'s of squid giant axons and of eel 
electroplaques last only about 2 msec. (figs. 3, 19), 
those of Aplysia giant neurons (fig. 7) or cat salivary 
glands (fig. 20) may persist for nearly i sec. The 
e.p.p.'s and p.s.p.'s of other neurons have inter- 
mediate durations. In .some cases, physostigmine 
and prostigmine both prolong the p.s.p., this effect 
probably involving the prolongation of the life of the 
transmitter, acetylcholine, by inactivation of cho- 
linesterase (cf. 52, 53, 60, 68). Some of the quater- 
nary ammonium compounds also prolong p.s.p.'s 
(cf 52) and these actions may be caused by direct 



172 



HANDBOOK OF PHVSIOLOGV 



NEUROPHYSIOLOGY I 



Fio. 20. Different types of electrical 
activity in cat salivary gland cells. 
Depolarization shown as downward 
deflection in these records. A: Type I 
cells produce hyperpolarizing p.s.p.'s 
which are graded with strength of the 
stimulus. Single shocks to chorda 
tympani evoke p.s.p.'s which last about 

I sec. B: Type I cells produce only 
hyperpolarizing p.s.p.'s to excitation ot 
the sympathetic {upper iigna!) or para- 
sympathetic {lower signal) nerves. How- 
ever, the latencies and magnitudes of 
the p.s.p.'s differ somewhat. C: Type 

II cells develop hyperpolarizing p.s.p.'s 
on stimulating the chorda tympani and 
depolarizing p.s.p.'s through their 
sympathetic innervation. D: Type III 
cells (which may be myoepithelial 
elements of the ducts) respond only with 
depolarizing p.s.p.'s to parasympa- 
thetic {above) or sympathetic {below) 
stimulation. The resting potential, 
about —80 mv, is large in comparison 
with that of Type I or II cells and 
resembles that of muscle fibers. E: 
Type I cells respond with hyperpolari- 
zation to epinephrine, acetylcholine 
and pilocarpine. [From Lundberg 
(144).] F: The hyperpolarizing p.s.p. 
of the gland cell is remarkably insensi- 
ti\e to changes of the membrane po- 
tential. The resting potential was 30 
mv. [From Lundberg (145).] 



B 



mV 

10+- 




Oh_ 


2sec 


-60 


- 




-50 


- 




-40 


- 




-30 


- 




-20 


- 






2 sec 




-40 




-120 
-80 
-40 
- 




2 sec 




-50 - 
-40 - 
-30 - 
-20 - 



2sec. 



l/^g adr. 



O.l/ig och. 




-120 
-100- 

- 80 
-60 
-40- 

- 20 

- 



0.5 /ig pilocar. 





effects upon the kinetics of the ionic 'valving' of tiie 
transducer action. 

The maximum attainable amplitudes of p.s.p.'s 
are probably determined by electrochemical condi- 
tions as described in a previous section of this 
chapter, but these need not be identical for different 
varieties of cells. Thus, most hyperpolarizing p.s.p.'s 
reach a limit set by the most negative electrochemi- 
cal ionic species, but hyperpolarizing electrogenesis 
of glands can occur in the face of very high internal 
negativity (fig. 20). These differences reinforce the 
conclusion (91, 105) that electrical activity of bio- 



logical membranes may involve a variety of mcch 
anisms, some of which are not yet understood. 

Postjunctianal Cells with Dfpolaii'ing 
Postsynaptic Potentials 

As noted above, some cells though not electrically 
excitaijle respond with depolarization to neural or 
chemical stimuli. Of general interest are electrically 
inexcitable invertebrate and vertebrate muscle 
hbers, such as the ' slow' muscle fibers of the frog 
(fig. 4i4). They are diffusely innervated and neural 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



173 



Stimuli give rise to graded summative depolariza- 
tions. These diffusely generated depolarizations can 
act as stimuli for the contractile mechanism, causing 
localized graded contractions (34, 35, 132). 

Some salivary gland cells also generate only de- 
polarizing p.s.p.'s (fig. 20) and these are produced 
by stimulation of either the sympathetic or para- 
sympathetic nerves (144). Chemical stimulation by 
epinephrine, pilocarpine or acetylcholine then all 
cause the same type of electrogenesis, but it is not 
known whether all the excitants activate a single 
variety of electrogenic membrane or whether there 
are distinct, although similarly electrogenic, cholino- 
ceptive and adrenoceptive components. As is the 
case with the electrically inexcitable muscle fibers, 
the synaptic electrogenesis of gland cells is also asso- 
ciated with and itself probably effects other cellular 
activity, in this case secretion.'^ 

Torpedo and Raia elcctroplaqucs also generate 
only depolarizing p.s.p.'s but not to electrical stimuli 
(95). The cells which are derived from skeletal 
muscles therefore are in reality constituted from end- 
plates. A specialization of the.se and other electro- 
plaques permits series additions of the voltages pro- 
duced by each cell; hence the electric organs generate 
considerable voltage. The discharges are under con- 
trol of the nervous system and in .some forms this 
may be useful for protection or aggression. The 
p.s.p.'s are Ijrief in Torj>edo but long-lasting in Raia. 

Vertebrate skeletal muscle fibers of the ' twitch' 
system and autonomic ganglia combine depolarizing 
p.s.p.'s and spike-generating membrane (cf figs. 9, 
27), but the autonomic neuron may also produce 
hyperpolarizing p.s.p.'s since there are indications 
thai inhil)ilion may occur (64, 134). In both cases 

' It was noted earlier (p. 154) that bioelectric responses of 
transmissional and conductile processes are essentially passive 
events resulting from the mo%ement of ions in obedience to 
charged electrochemical equilibrium states. The change from 
one state to another is the active phenomenon, due to specific 
processes, transducer actions which are the responses of excit- 
able membrane to appropriate stimuli. In gland cells, the se- 
cretory activity of the output component (fig. 16) probably 
occurs at membrane .structures that are intimately mingled 
with those of the input component. .Secretory electrogenesis 
thus is probably superimposed on the p.s.p.'s of the transducer 
input, and this is suggested also by the independence of the 
gland electrogenesis from electrochemical conditions (fig. 20 
E and F). In some respects, therefore, electrogenesis of gland 
cells may differ from that of 'pure' p.s.p.'s of neurons or end- 
plates. The details of these diflferences cannot now be specified 
since little is known about the nature of active transport 
mechanisms, such as arc probably involved in secretion. 



the p.s.p.'s have much longer durations than do the 
spikes. In muscle fibers the spike energizes the proc- 
esses of contraction by a mechanism that is not yet 
known (cf 121). Eel electroplaques also generate 
both depolarizing p.s.p.'s and spikes (cf figs. 3, 6, 
13), but the contractile machinery is missing in these 
modified muscle fibers. Eel electroplaques, like 
neurons, are diffusely innervated Isy many nerve 
fibers. Since the area of their innervated surface is 
more than 10 mm-, their study has provided some 
data that are not readily obtained with the much 
smaller nerve cells. The results, however, very prob- 
ably apply to the general case of synaptic transmis- 
sion as will be described below (cf. 95). 

Postjunctional Cells with Hyperpolarizing 
Postsynaptic Potentials 

If cells capable of generating spikes were endowed 
only with hyperpolarizing p.s.p.'s, transmfssional 
excitation of the electrically excitable responses would 
not occur, for in all cases known the spike is triggered 
by depolarization. Thus, it may be expected that 
cells in which solely hyperpolarizing synaptic elec- 
trogenesis occurs would be of restricted functional 
significance. From intracellular recordings two cases 
are known and in neither are spikes generated. These 
are salivary gland cells (144, 146; cf fig. 20) and L- 
cells of the fish retina (102, 184, 191 ; cf al.so footnote 
4, above). As noted earlier, the memijrane trans- 
ducer actions and electrochemical effects of hyper- 
polarization are consistent with secretory activity; 
hence neurally evoked hyperpolarizing p.s.p.'s of 
glands have functional validity. 

Postjunctional Cells ivith Both Types of 
Postsynaptic Potentials 

Two varieties may be expected and both types 
occur: /) electrically ine.xcitable cells which do not 
generate spikes and 2) cells which produce spikes as 
well as the p.s.p.'s. A clear case of the former is 
found in some salivary gland cells (fig. 20) in which 
each type of synpatic electrogenesis is probably asso- 
ciated with a different form of secretory activity. The 
different p.s.p.'s are specifically produced by stimu- 
lation of the two autonomic nerve supplies. Stim- 
ulation by cholinomimetic and adrenomimetic 
substances evokes oppositely signed electrogenesis. 
The R-G and Y-B cells of fish retina also produce 



174 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



depolarizing and hyperpolarizing potentials without 
spikes (184). 

Some invertebrate muscle filjers possess dual 
synaptic activity (72, 73) and it has been suggested 
(96) that vertebrate smooth muscle may also belong 
to this category. If the fibers are not electrically 
excitable, the contractions caused by their depolariz- 
ing p.s.p.'s would be local, as in frog "slow' muscle 
fibers (34, 35, 132). The hyperpolarizing p.s.p.'s 
would serve the function of diminishing or regulating 
the degree of the mechanical response by decreasing 
the depolarizations of the 'excitatory' p.s.p.'s. 

By far the most prominent class are the cells in 
which spikes as well as the two kinds of p.s.p.'s are 
generated. Most, and perhaps all, neurons of the 
vertebrate central nervous system probably belong 
to this group (cf. 59, 60, 158, 159, 161 -167). The 
hyperpolarizing and depolarizing p.s.p.'s appear to 
have nearly identical durations and the superposi- 
tion of the two p.s.p.'s may decrease membrane 
depolarization sufficiently to eliminate spike produc- 
tion by an orthodromic excitatory pathway. This 



interaction of depolarizing and hyperpolarizing 
p.s.p.'s adds to the variety and flexibility of integra- 
tive activity within the central nervous system. The 
effects are achieved not only by relatively simple 
algebraic summation of the potentials but also by 
the interplay of more subtle factors which will be 
described in the next section of this chapter. 

Fast and Slaw Respunses of Iinrrtebratt' Muscles 

The muscle fillers of some insects and Crustacea 
(cf. 116, 117) are known to be electrically excitable, 
but they also respond diff^erently to stimulation of 
different excitatory nerves (fig. 21). Their 'fast' in- 
nervation, which may be constituted of one or several 
nerve fibers, produces large depolarizing p.s.p.'s 
upon which is superimposed a spike-like response, 
often showing a small overshoot. Stimulation of the 
'slow' nerve fiber leads to a small depolarizing p.s.p. 
Upon this there may develop various gradations of 
the electrically excitable response. The mechanical 
acti\ities are also different. The fast nerve fiber 



mV 




FIG. 21. Different responses produced in insect muscle fibers on stimulating their fast and slow 
innervation. Intracellular recording from e.xtensor tibiae of the mesothoracic leg of Schistocerca gri'- 
garia. A: The responses of six different muscle fibers, first to stimulation of the fast nerve fiber and 
then the slow. In all but one muscle fiber, the fast response developed an overshoot. .^K notch on the 
response of fiber ii indicates the level on the p.s.p. out of which the spike-like activity developed. 
In fiber ;, as in about 50 per cent of the muscle fibers, no response resulted on stimulating the slow 
nerve fiber. Various grades of activity are shown in the other examples. In three of these (iii, iv, r) 
the p.s.p. was large enough to evoke some local response of the electrically excitable membrane. B: 
Three examples of facilitation of the p.s.p.'s by repetitive stimulation of the slow nerve at about 
30 per sec. The augmented p.s.p.'s evoked larger pulsatile local responses, and in one case (i.v) an 
overshoot was obtained. Time and amplitude calibration in (0 apply to all records. [From Hoyle 
(1 16).] 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



'75 



evokes a brisk twitch or a maximal tetanus. The 
slow fiber calls forth small contractions which may 
grow slowly during repetitive stimulation. 

The apparent paradox that depolarization, an 
electrical and unspecific stimulus, can evoke differ- 
ent forms of response in electrically excitable mem- 
brane has been resolved by the finding (37, 38) that 
the membrane of the muscle fibers of the grass- 
hopper, Romalea microptera, though electrically ex- 
citable, responds only with graded activity. Other 
physiological and anatomical circumstances co- 
operate with this normally occurring graded respon- 
siveness. The different nerve fibers evoke two degrees 
of depolarizing p.s.p.'s in the electrically inexcitable 
synaptic membrane. The p.s.p.'s evoked by the fast 
nerve fiber may be larger because of greater synaptic 
potency of the 'fast' transmitter system than in that 
of the slow fiber (e.g. a different agent, a higher 
concentration of transmitter, closer approximation 
of the pre- and postsynaptic membrane or larger 
area of synaptic contact). However, another alterna- 
tive is that the membrane sites engaged by the 
terminals of the different fibers are different. The 
combination of graded p.s.p.'s and electrically 
excitable local responses is abetted by the closeness 
of synaptic terminations. The terminals of the fast 
nerve fiber, spaced as close as 40/j apart, can each 
evoke large local responses of the electrically ex- 
citable membrane. This graded activity, summing 
its depolarizing actions, can then evoke maximal re- 
sponses which have the appearance of spikes. The 
associated contraction is a twitch. The smaller 
p.s.p.'s of the slow response can be graded in various 
proportions and can evoke local response of various 
degrees. The resulting contractions are also graded. 

The mechanisms involved in the dual responses 
of muscle fibers are instructive for several reasons. 
Dual responsiveness is probably present in muscles 
of animals quite low in the evolutionarv scale (i 17), 
and this suggests that electrically excitable mem- 
brane, like the sen.sory or synaptic, was originally 
gradedly responsivp. The ability to develop spikes 
then would have been a later evolutionary stage (2 1 , 
103). Dual responsiveness also represents an ex- 
ceedingly useful mode of activity for arthropods for 
their muscles are limited in number. The size of the 
muscles and therefore also the numijer of their fibers 
are limited by the exigencies of the exoskeleton. The 
number of nerve fibers is also rather small. Despite 
these limitations arthropods can manipulate their 
joints intricately and with precision and carry out 
locomotion with great dispatch and vigor. These 



different aspects of movement are all achieved with 
an economy of means because of special responsive 
mechanisms and anatomical conditions. 



PHARMACOLOGICAL PROPERTIES OF SYNAPSES 
.^ND THEIR PHYSIOLOGIC.-VL CONSEQUENCES 

The discussion in this part of the present chapter 
will be limited to vertebrate synapses, concerning 
which information is more extensive than on in- 
vertebrate structures. However, the pharmacology 
of the electrically inexcitable sensory membrane of 
the crayfish stretch receptor probably parallels that 
of synapses in the cat brain (96). This suggests 
that in their general aspects the pharmacological 
properties of vertebrate and invertebrate synapses 
will be similar in principle, although, perhaps, 
invoKing different chemical suijstances. In crustacean 
neuromuscular synapses and in the inhibitory 
synapses of the stretch receptors the actions of amino 
acid drugs parallel to a degree the effects of these 
substances in cat brain (cf. 99, 163, and below). 
However, other invertebrate synapses appear to 
have no pharmacological relation to vertebrate 
synapses (cf. 99 j. 

Classification of Drug Actions 

Depending upon the theoretical approach and the 
experimental emphasis, several varieties of classifica- 
tion have arisen. Thus, drugs have been grouped as 
'mimetics' or 'lytics', graded according to the degree 
to which they mimic or block the action of nerve 
impulses, or sometimes of a standard comparison 
substance (cf. 8). Particularly in describing effects of 
drugs on the more complex synaptic systems (chiefly 
of the central nervous system but also those of smooth 
muscle) substances have been classified as 'excitants' 
(or 'stimulants') and 'inhibitors' (or 'depressants'). 
For example, since both pentylenetetrazol (Metra- 
zol) and strychnine are convulsant agents, both 
are classified as stimulants of the central nervous 
system (cf. 85). Recently (cf. 156) the drugs acting 
upon the peripheral cholinoceptive synapses of 
skeletal muscle and autonomic ganglia have been 
classified as 'depolarizing' or as 'nondepolarizing, 
competitive, antagonistic inhibitors' of the latter. 
This classification also applies to the simple depo- 
larizing synapses of the eel electroplaques (table 2). 

An extension of this classification (table 3) has 
proved experimentally and analytically more useful 



176 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



TABLE 2. Range of Effectiveness on Single Eel 
EleUroplaques of Some Synapse Inactivating and 
Synapse Activating Drugs 

Minimum effective 
concentration 
Substance '" fg per ml 

a) Compounds which inactivate the postsynaptic membrane 
of eel electroplaques, do not depolarize, but convert the 
all-or-nothing response of the electrically excitable 
membrane to the gradedly responsive 
Physostigmine 25 

</-Tubocurarine 50 

DFP* 100 

Procaine 200 

Tertiary analog of prostigmine 1000 

Flaxedilt 

A) Compounds which activate synapses of eel electroplaques. 
The resultant depolarization secondarily inactivates the 
electrically excitable membrane. Synaptic electrogenesis 
still occurs 

.\cetylcholinet 5 

Carbamylcholine 10 

Decamethonium 10 

Dimethylaminoethyl acetate (DMEA)t 50 

Prostigmine 5° 

SuccinylcholineK 

* This substance causes a secondary depolarization with 
consequent inactivation of the electrically excitable mem- 
brane. 

t Included on the basis of the data of Chagas & Albe- 
Fessard (39) that the action of Flaxedil is similar to that of 
curare. These workers did not study membrane potentials or 
graded responsiveness. Chemically Flaxedil is tri-(diethyl- 
aminoethoxy) benzene triethyliodide. 

I In the presence of 25 ^g per ml physostigmine. 

Tl On the basis of the data of Chagas & Albe-Fessard 
C39). who found a similarity of action with acetylcholine 
(see note f). 



since it applies as well 10 hyperpolarizing synapses 
and to systems containing both electrogenic types 
(96, 97). The two major varieties of drugs are in this 
case classified as activators or inacti\ators of synaptic 
electrogenesis. The nature of the latter, depolarizing 
or hyperpolarizing, is determined only by the type 
of synapse not by the activator substance. Each major 
group is subdivided into drugs which act nonselec- 
tively or selectively upon either the depolarizing or 
hyperpolarizing synapses. The interactions of drugs 
and synapses disclose many sui)sidiary classifications, 
both in the drugs and in synaptic membranes (99, 
100, 108), but these need not be considered here. 

The overt manifestations of 'excitation' and 'in- 
hibition' of the six classes of drugs in table 3 need 
not correspond to the basic mode of achieving this 
effect at the synaptic level. Thus, the 'excitant' ac- 



TABLE 3. Possible Combinations of Actions of 
Synaptic Drugs 





Effect 


Synapses Affected 


Overt .Action 




Agent 


Depola- 
rizing 

(Excita- 
tory) 


Hyper- 
polar- 
izing 

(Inhibi- 
tory) 


Type Compound 


.Activators 


2 

3 


+ 
+ 



+ 


+ 


Excitation 
Excitation 
Inhibition 


Acetylcholine 
Metrazol 


Inacti\ators 


4 
5 
6 


+ 
+ 



+ 


+ 


Inhibition 
Inhibition 
Excitation 


Curare 
GABA 

Strychnine 



-f indicates an effect; o, none. Diphasic actions omitted. 



TABLE 4. Cortical Synaptic Actions of Aliphatic 
Amino Acids 



Car- 
bon 
No. a-amino acids 



w-ammo acn 



ids 



Glycine 



3 o 

(a-alanine) 



)-diamino acids 
X 

X 



(/3-alanine) 
(7-aminobutyric) 



(a-aminobu- (7-aminobutyric) (2,4diamino- 
tyric) butyric) 

50 -- o 

(Norvaline) (a-amino ly-valeric) (Ornithine) 
6 o +++ + 

(Norleucine) (e-amino caproic) (Lysine) 
8 X -f-|- + + X 

(co-amino caprylic) 

Symbols: — to indicate increasing blockade of 

excitatory synapses which leads to overt inhibitory' action; 
+ to -|- + -|--|- represent increasing blockade of inhibitory 
synapses leading to 'excitatory' effects; o, compound not 
active; X, not available or not tried. 



tions of the two conxulsant agents, strychnine and 
pentylentetrazol, are produced by entirely difTerent 
fundamental processes (166). The similarities in 
overt cfl'ects arise from the conditions that prevail in 
systems which contain many synapses and of both 
tvpes. It is then likely that an activity is a mixture 
involving both excitatory and inhibitory synaptic 
actions, and the study of the central nervous system 
has revealed many examples of this. Blockade of 
synaptic activity thus becomes a positive act, en- 
hancing or diminishing overt manifestations such as 
motor activity, depending upon which type of 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



■77 




FIG. 22. Synaptic actions of shorter-chain u-amino acids. 
Column A shows the response evoked in the cat cerebral cortex 
by a local electrical stimulus (five superimposed traces indicate 
the degree of variability). The surface negative potential (up- 
ward deflection) is the p.s.p. of superficial dendrites. B shows 
the effects of applying 0.2 cc of a i per cent buffered w-amino 
acid. The substances are identified by the letters on the left 
which correspond to those in table 4. .'Ml the compounds in- 
verted the surface negativity to a surface positi\'ity by blocking 
production of depolarizing p.s.p.'s and thereby disclosing hy- 
perpolarizing p.s.p.'s which are recorded as surface positivity 
(downward deflection). The action of all four substances was 
similar but differed in magnitude and rate of onset, both factors 
being largest with C, (GAB.A). Column C shows that recovery 
from the action of the compounds is seen 3 min. after rinsing 
the cortical surface several times with Ringer's solution. Time 
at bottom, 20 msec. [From Purpura el al. (163).] 

synapse is inactivated. Il is for this reason that 
selecti\e Ijlockade of inhibitory synapses by strych- 
nine leads to excitatory' actions, augmented elec- 
trical activity or convulsions. 

The selective action of many drugs on either hy- 
perpolarizing or depolarizing synapses introduces an 
important factor. A substance may act powerfully 
on one synaptic .system and yet be inert with respect 
to another which lacks the appropriate synaptic 
substrate for the drug. This has been experimentally 
verified with strychnine which is a highly selective 
inactivator of hyperpolarizing inhibitory synapses 
(fig. 14). Strychnine is inert, except in very high 
concentrations, on structures like the muscle end- 



plate or the vermian cerebellar cortex of cat. How- 
ever, when given in high concentration it does act 
to blockade the depolarizing excitatory synapses 
C166). 

In view of the foregoing, tests on relatively simple 
synapses (table 2; figs. 11, 13, 15) may not be ade- 
quate for analyzing drug actions. This fact is illus- 
trated by the recent demonstration and analysis of 
the synaptic actions of various amino acids (162, 
163). The a)-amino acids tested (table 4), substances 
in which the amino group is on the terminal carbon 
farthest from the carboxyl radical, are selective in- 
activators of cortical synapses. The shorter chain 
compounds (C2 to C5, fig. 22) block depolarizing 
activity of the dendrites while compounds Ce and 
Cs (fig. 23) inactivate hyperpolarizing synapses. 

One of these substances, 7-aminobutyric acid 
(GABA), occurs naturally in the brain (12, 173) and 
has been identified (16) as a component of the 'in- 
hibitory factor' which can be extracted from mam- 
malian brain and which diminishes the discharge 
of impulses in the mechanically excited crayfish 
stretch receptor. As a selective blockader of de- 
polarizing receptor and synaptic membrane, GABA 
can only act as an ostensible 'inhibitor' when con- 
fronted with the simple depolarizing electrogenic 
membrane. Thus it acts on the cerebellar cortex as 



CAPROIC (Cg) 




FIG. 23. The qualitatively difl"erent effects produced by 
w-amino acids with longer carbon chains. In each experiment, 
responses were simultaneously evoked from the surface of the 
cerebral cortex (upper trace) and cerebellar cortex (lower 
trace). / and 4 show the responses in different experiments be- 
fore applying the amino acids; 2 and 5, the cerebral p.s.p.'s 
increased on applying C^ or Cg. The cerebellar activity was not 
affected indicating that these u-amino acids are inert toward 
the cerebellum. 3 and 6, responses after rinsing the cortical 
surfaces with Ringer's solution. Time, 20 msec, is different in 
the two experiments. Four traces superimposed in each record. 
[From Purpura et al. (162).] 



■78 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



it does on crayfish stretch receptor membrane by 
eHminating the depolarizing electrogenesis (fig. 24). 
However, when GABA is applied to the cerebral 
cortex, its selective elimination of depolarizing 
surface-negative p.s.p.'s discloses the previously 
masked hyperpolarizing surface-positive p.s.p.'s. 
Acting in the cerebral cortex (figs. 22, 24) GABA 
and its congeners invert the electrocortical activity 
evoked by a stimulus. 

The effects of the .selective inactivators of hyper- 
polarizing synapses, Ce and Cg (fig. 23), also differ 
depending upon the type of electrogenic structure 




20 MSEC 



FIG. ^4. Different effects of the selective inactivator of de- 
polarizing p.s.p.'s at different sites. A, t to 5.- Simultaneous re- 
cordings from the cerebral cortex with a large surface electrode 
(upper trace) and a fine wire electrode (lower trace). /, both 
electrodes were on the surface and recorded nearly identically 
the evoked surface negative p.s.p.'s of the superficial cerebral 
dendrites. 2, the fine electrode was inserted about 0.4 mm be- 
low the surface into an essentially isoelectric region. 3 and 4, 
application of GABA to the cortical surface reversed the surface 
response into positivity, but this change did not appear in the 
subsurface recording. This indicates that the effect produced by 
the amino acid was on superficial p.s.p.'s only, j, rinsing the 
cortical surface restored the original activity at the surface. The 
subsurface recording was still unchanged. 6, superimposed 
responses before and during the action of G.^B.'K. B: The simul- 
taneous recordings in this experiment were from the cerebral 
cortex (upper trace) and the cerebellar (lower trace). /, before 
applying GABA; 2, five drops of 0.1 per cent G,'\B.'\ were ap- 
plied to each site. In the cerebral cortex the result was a reversal 
of surface potential. In the cerebellar cortex the surface nega- 
tivity was eliminated by blockade of the depolarizing p.s.p.'s, 
but no positivity developed because of the paucity of hyper- 
polarizing synapses in this structure. 3, recovery was rapid in 
the cerebral and slower in the cerebellar cortex. Time, 20 msec. 
[From Purpura et al. (163).] 



that is used as a test object. Neither the crayfish 
stretch receptor nor the cerebellar cortex is affected 
by application of co-aminocaprylic acid (Cj). How- 
ever, the surface negativity evoked in the cerebral 
cortex is augmented by the blockade which Ce and 
Cs cause amongst the surface-positive p.s.p.'s of the 
hyperpolarizing synapses. 

Recent work (Grundfest et al., in preparation; cf. 
99, 163) indicates that the axodendritic synaptic 
membrane in the cat brain stands in a doubly in- 
verted pharmacological relation with some crusta- 
cean synapses. GABA and other inactivators of the 
cat depolarizing synapses activate crustacean in- 
hibitory synapses. Picrotoxin, an activator of cat 
excitatory synapses, inactivates the crustacean 
inhibitory synapses. One of the selective inactivators 
of cat inhibitory synapses, carnitine (cf. 163), activates 
the excitatory synapses of lobster muscle fibers. 
However, these inverted parallels are not complete. 
The Ce and Cs co-amino acids do not affect the 
crustacean synapses. Likewise, acetylcholine, d-luho- 
curarine and strychnine are without effect. 

In sum, it may be concluded from the foregoing 
discussion that determination of the mode of action 
of a drug depends not only on the degree of intimate 
knowledge which may be obtained of its synaptic 
effects but also upon the type of information that 
may be provided by the test object. The synaptic 
structure u.sed for the tests may be too complex to 
yield the details required, but also it may be too 
simple and provide only misleadingly partial in- 
formation. 

Identification and Characterization oj Transmitter Agents 

The preceding section sets the theoretical and 
methodological background for the problems treated 
in this. The quantity of transmitters released during 
activity of presynaptic terininals is probably ex- 
ceedingly small (cf. 52, 59, 60, 68). The problem of 
their identification therefore is strongly conditioned 
by methodology. For example, norepinephrine has 
been known, since its laboratory synthesis in 1904 
(cf. 193), to have properties similar to those of its 
homologue, epinephrine. Also, the work of Cannon 
and his associates (cf. 1 77) had indicated very 
clearly that there must be at least several sym- 
pathetic transmitters which were designated as 
sympathins E (excitatory) and I (inhibitory). Never- 
theless, norepinephrine was not accepted as a pos- 
sible sympathetic transmitter until it was shown in 
1946 (cf 193) that it is a natural constitutent of the 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



'79 



body. Likewise, interest in GABA stems from the 
demonstration of its occurrence in the brain in an 
important pathway of synthesis (12, 172). Thus, the 
candidate for a transmitter agent must meet a num- 
ber of requirements (cf. also 65): a) it must mimic 
closely the actions produced by the natural, neural 
stimulus; b') its actions must be affected by the same 
drugs and in the same ways as neural excitation is 
modified; c) it must be a naturally occurring con- 
stitutent, found in close proximity to the relevant 
synaptic structures; and (/) it is desirable to demon- 
strate that it is formed by an appropriate metabolic 
pathway, that it is released at the time, place and in 
the degree suitable to transmitter action and that its 
accumulation to excess is prevented by another 
metabolic pathway. 

Characterized by the foregoing criteria, acetyl- 
choline and the catechol amines of the epinephrine 
group are still the only substances commonly agreed 
upon and accepted as peripheral transmitter agents. 
Most conspicuously, these substances derive their 
claim to transmitter agents by their actions as 
synapse activators. Thus, acetylcholine is probably 
the excitatory transmitter at electroplaques, muscle 
fibers, autonomic ganglia and some gland cells. At 
the effector junctions of the cardiac pacemaker and 
probably also in many smooth muscle systems (96), 
acetylcholine activates hyperpolarizing synapses and 
is inhibitory. The epinephrine group of transmitters 
acts similarly at other synapses. However, these 
transmitters also appear to have an accessory func- 
tion (cf. 36). Thus epinephrine may antagonize the 
action of decamethonium (47) or relieve ' fatigue' of 
neuromuscular transmission upon repetitive stimula- 
tion (119).^ 

In complex synaptic systems, one may as.sign 
transmitter action to substances which do inac- 
tivate synapses. For example, GABA is a synapse in- 
activator, but if it is released by specific nerve fibers 
its effects would be essentially inhibitory — with the 
important exception that there would be no accom- 
paniment of hyperpolarizing p.s.p. Likewise there 
might be transmitters, analogous to Cg, whose overt 

^ Neuromuscular blockade by decamethonium is a manifes- 
tation of Wedensky inhibition discussed earlier. Antagonism by 
epinephrine suggests that this transmitter agent acts as a com- 
petitive antagonist, or synapse inactivator, of cholinoceptive 
synaptic membrane. This type of action is apparently contra- 
dicted by the ciTect of epinephrine in lifting the blockade pro- 
duced by repetitive activity. However, there need be no real 
contradiction for synaptic membrane may change its properties 
under different experimental circumstances, an indication of 
the complexity as well as lability of the active structure (cf. 96). 



action, excitation, might be produced by inactivat- 
ing hyperpolarizing inhibitory synapses. 

These considerations indicate the difficulty of 
identifying transmitters in a complexly organized 
synaptic structure. The difficulty is enormously com- 
pounded in the central nervous system, where even 
a small volume of tissue contains a huge number of 
synapses. In such a case all the criteria for categoriz- 
ing transmitters cannot be fulfilled at present and 
therefore identification is always tentative, based as 
it must be on incomplete evidence. 

Nevertheless, there is evidence from various sources 
that acetylcholine and the adrenergic agents do af- 
fect central nervous activity. Thus, circulatory injec- 
tions of epinephrine (22) or acetylcholine (cf. iii) 
bring about EEG activation as does stimulation of 
the peripheral stump of the cat splanchnic nerve 
(22). The electrical activity of a cortical slab, iso- 
lated from its neural connections but surviving with 
intact blood supply, is altered upon electrical stimu- 
lation of the brain stem reticular formation (122). 
Thus, brain stem activity releases some transmitter 
agents which can then affect the isolated cortex. 
This finding has been extended to cross-perfused 
preparations (160). From that work it may be con- 
cluded that what is released during brain stem ac- 
tivity enters the systemic circulation and that it 
must be a substance (or several) more stable than is 
acetylcholine. The latter probably would have been 
destroyed completely or almost so during the time 
required for an exchange of circulating blood be- 
tween donor and host. Many workers have shown 
that acetylcholine is found in the central nervous 
system as well as its synthesizing acetylating enzyme 
(for references to the recent literature cf. 65). The 
distributions of these substances in the brain and of 
sympathetic transmitters (192) have also been 
mapped. Lesions in some regions of the reticular 
formation augment or depress the sensitivity of the 
cortical electrical activity to epinephrine (178) 
Intraventricular application of cholinomimetic and 
adrenomimetic substances or of blockaders of the 
two types of synapses produce a variety of central 
nervous symptoms (cf. 75, and literature cited there). 
Intravenous injections of (/-tubocurarine block cen- 
tral nervous synapses (cf. 165). 

Evidence with respect to other agents is still in- 
conclusive. Although 5-hydroxytryptamine (sero- 
tonin) and metabolically related substances are be- 
lieved by some to be implicated in transmission, 
whether they act directly or not is still in question 
(cf. 26, 128, 148). As stressed earlier in this part. 



i8o 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



the difficulties are largely methodological because 
central synapses are so intricately interrelated and 
may present many varieties. Stemming from this is 
the difficulty of determining whether or not various 
synaptic sites are afifected, and if they are what 
results are to be looked for. Thus it has been shown 
(162, 163) that the synaptically active amino acids 
affect primarily only axodendritic synapses of the 
cortex and only secondarily the axosomatic. Some 
consecjuences are seen in figure 25. Blockade of ex- 
citatory synapses of the dendrites by C4 (GABA) or 
of inhibitory synapses by Ce (e-aminocaproic acid) 
does not afTect the corticospinal discharge of the 
pyramidal cells. However, the convulsive electro- 
cortical activity induced by Cs (oj-aminocaprylic 
acid) leads to prolonged discharge in the tract. 

A further difficulty is the problem of accessibility 
of the central synapses to testing drugs. The blood- 
brain barrier apparently is highly effective for some 








FIG. 25. Pyramidal tract activity when dendritic responses 
in cerebral cortex are affected by u-amino acids. Column A, the 
discharge recorded from the pyramidal tracts to stimulation of 
the cerebral cortex in cats. Then, 0.2 cc of i per cent u-amino 
acid had been applied for 10 min. The substances were C4 and 
Cs as noted in each row of records. The responses of column B 
were obtained when the cortical potentials had been altered as 
shown in figs. 22 and 24. Despite these changes, the pyramidal 
tract responses, generated by direct electrical stimuli and by 
axosomatic synaptic excitations, were not affected (C4, Ce) 
except when as in the case of C^, the drug caused convulsions. 
Then a long after-dischsirge, associated with the convulsions, 
developed. Ten superimposed traces in the upper records, five 
in the middle and lower set. Time 10 msec. [From Purpura et 
a/. (163).] 



substances, e.g. GABA [Roberts & Baxter (172)]. 
Recent experiments (164) demonstrate that local 
abolition of the blood-brain barrier permits the local 
action of systemically injected oj-amino acids. These 
results indicate that if the substances are elaborated 
within the brain they might act as transmitters (using 
the term for both synapse activators and inactivators; 
cf. above), although the usual experimental criteria 
would not disclose such action. 

Modes of Action of Transmitter Agents and Synaptic Drugs 

Since transmitters must be formed and, after their 
release, metabolized in the body, enzymes for these 
activities are components more or less related to the 
appropriate synaptic systems. In the search for 
mechanisms of drug action, interference with en- 
zymatic or other metabolic processes has been fre- 
quently stressed (cf. 2, 13, 14, 26, 65, and literature 
cited in these papers). Undoubtedly-, interference 
with these metabolic systems must cause synaptic 
disturbance; but it is likely that such actions are 
relatively slow, manifesting themselves, as in the 
case of vitamin deficiences, only after depletion of 
reserves. This is not the case with drugs that have 
primary action on synapses (108). This may be .seen 
in figures 22 to 24 in which the synaptic effects of 
some of the oj-amino acids were obtained within a 
second after they were applied and were rapidly 
reversed by dilution. 

Furthermore, substances of the same type of action 
on enzymatic systems may have entirely different 
synaptic actions. Thus physostigmine, DFP and 
prostigmine are powerful inhibitors of cholinesterase. 
By that effect all three, in very high dilution, enhance 
neural action in eel electroplaques (5). This is merely 
the indication that they prolong the life of the labile 
transmitter agent. However, the synaptic actions of 
the three drugs on the eel electroplaque are diverse 
(table 2). In that capacity prostigmine is a synapse 
activator like acetylcholine itself Physostigmine js an 
inactivator like (/-tubocurarine and about as potent 
in that effect. DFP appears to have dual actions 
such as are to be found in the many other situations 
(cf. 95, 96, 127). 

The conclusion reached from these considerations 
leads back to the view first proposed by Ehrlich (cf 
2, 14, 40) that drugs exert their action by affecting, 
perhaps by some form of chemical or electrostatic 
combination, the performance of specific molecular 
structures of the cell membrane. This receptor theory 
has had many vicissitudes, apparently largely be- 
cause of static conceptions of such functional units. 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



lai 



Recently the models examined have been endowed 
with dynamic properties (cf. 2, 9, and literature 
cited in the papers). These current theoretical 
formulations have had some success in accounting 
for relations between structures of drugs and their 
functions. They do not, as yet, consider the implica- 
tions of the recent findings concerning specificity of 
drug action on one or the other type of synaptic 
membrane. Thus the addition of one carbon link to 
an co-amino acid converts a substance which is pre- 
dominantly an inactivator of depolarizing synapses 
(Cs) to another (Ce) which inactivates chiefly, or 
perhaps exclusively, the hyperpolarizing type (table 
4; figs. 22, 23). In other relations of drugs, similar 
abrupt transitions depending upon number of 
carbons (the transformation occurring at about five 
carbon.s) have also been noted (cf 14, p. 147). 

The occurrence of distinct varieties of synaptically 
acting chcmotherapeutic agents, e.g. analgesics, 
antipyretics, etc., bespeaks relatively sharp, though 
not absolute, difl^erences between synaptic mem- 
branes in differently acting regions of the central 
nervous system. Similar distinctions, both peripheral 
and central, derive from the relatively specific actions 
of other drugs. Thus, whether synaptic transmission 
is blockaded by atropine or by fZ-tubocurarine forms 
part of the differentiation between muscarinic and 
nicotinic cholinoceptive synapses. 

Ph ysio/og ical Im plica lio n s 

Only a few selected aspects can be discussed here 
of the relations between the modes of action of trans- 
mitter agents and their physiological consequences. 

a) topographic distinctions. In many cases the 
action of a transmitter must be rather strictly lo- 
calized. This is due to a number of factors which 
differ in importance for diflferent transmitters and 
synaptic sites. The small quantity of transmitter re- 
leased by a presynaptic nerve fiber would rapidly 
lose effectiveness upon diffu.sion and dilution in the 
volume away from the synaptic site. It may be 
destroyed by enzymes or fLxed in various chemical 
combinations. Its effectiveness at other synaptic 
sites may be small or absent. The rate at which it 
moves from the region in which it was liberated may 
be very slow. 

These, and other factors that may be postulated, 
tend to restrict transmitter action to limited sites, 
although under special experimental conditions dif- 
fusion is easily demonstrated (cf 177). Electrical 
ine.xcitability of .synaptic membrane and its chemical 



specificities promote restriction of transmitter action 
which is desirable in intricate synaptic relations. The 
specificities of different, perhaps of alternating 
synapses in a .synaptic sequence, as suggested by 
Feldberg (cf 157), would be one means of achieving 
this result. In the spinal cord, interneurons and 
motoneurons appear to have somewhat different 
pharmacological properties (cf. 60). 

Diff'erent parts of the same neuron might also have 
differently sensitive synaptic membranes. Thus, the 
co-amino acids appear to be chiefly effective as 
synapse inactivators at the superificial axodendritic 
synapses of cortical neurons (fig. 25). In the context 
of electrically inexcitable activity of these dendrites 
(165) the function of dendritic electrogenesis is prob- 
ably that of modulating somatic responsiveness, a 
consequence which cannot be discussed here (cf. 

lOl). 

b) synaptic specificity and transmitters. Eccles 
(cf. 60) has emphasized the implication of Dale's 
suggestion (46) that one neuron at all its profuse 
terminals probably generates only one type of trans- 
mitter. This ' principle' is reasonable but is not at 
all an obligatory condition. Furthermore, a neuron 
secreting the same transmitter at different synaptic 
sites may produce depolarization and be an 'ex- 
citant' at one, or cause hyperpolarization and be an 
'inhibitor' at another variety of synaptic membrane. 
Likewise, the same neuron might produce at its 
different terminals several varieties of transmitters 
which might all have the same effect, excitatory or 
inhibitory, or opposite actions, depending entirely 
upon the variety of postsynaptic membrane which is 
in synaptic relation with the transmitters. This 
emphasizes that the nature of the transmitter can 
determine synaptic potency and the kinetics of the 
synaptic activity (cf 97). The type of electrogenic 
action is determined by the postsynaptic membrane.' 

c) reciprocal interactions of neural pathways. 
The mechanisms of dual action discussed above have 
bearing upon the interpretation of reciprocal innerva- 
tion. r Sherington discovered in spinal reflexes (44) 
that the development of reflex activity in one muscle 
is associated with concurrent inhibition of antagonis- 
tic muscular activity. These interactions extend to 

' Interactions of some drugs evoke apparently dual actions 
at the muscle endplatc (cf. 53). These may be cases of the situa- 
tion commented upon earlier, in which a drug activates some 
components and inactivates others in the same synaptic mem- 
brane. This implies that the membrane of a single synapse is 
not homogeneous. 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 1 



Other muscle groups that participate in an organized 
movement (cf. 140). These effects frequently are 
fully reciprocal, excitation in either path being asso- 
ciated with inhibition in the other, and thus they 
involve processes of reciprocal inhibition as well as 
primary excitation. 

A mechanism, discovered by Lloyd (cf. 138) and 
termed direct inhibition, was believed to be mediated 
monosynaptically, the collaterals of the same afferent 
nerve fiber at one motoneuron evoking excitation, 
tho.se to another producing inhibition. This mecha- 
nism would imply that the same transmitter evokes 
depolarizing p.s.p.'s in one neuron and hyperpolariz- 
ing p.s.p.'s in another cell. From the point of view of 
theoretical considerations this means of achieving 
reciprocal actions is perfectly feasible, as is the possi- 
bilitv that different transmitters are released at the 
different terminals of the same primary afferent nerve 
fiber. In the lobster cardiac ganglion (33, 186) one 
presynaptic nerve causes excitation in one neuron 
and inhibition in another. 

The reality of monosynaptic direct inhibition of 
this type in the cat spinal cord is at present in dis- 
pute. Lloyd and his colleagues (cf. 194) maintain 
that a monosynaptic pathway exists while Eccles 
and his associates (cf. 60) consider that 'direct' in- 
hibition is a disynaptic event. Whether the particu- 
lar reflex pathways under discussion are mono- 
synaptic or disynaptic is probably a matter of the 
specific structures involved and perhaps of the 
methodological details.* In principle, direct inhibi- 
tion by monosynaptic reciprocal innervation can 
occur. Since it is theoretically feasible it seems un- 
likely that among the many types of connections 
elaborated in the nervous system one possible and 
rather simple variety has been omitted. 



ROLE OF ELEMENT.-^RY SYN.\PTIC PROPERTIES 
IN INTEGRATIVE ACTIVITY 

Spatial Interrelations of Synaptic and 
Conductile Aiembrane 

Since p.s.p.'s are 'standing' nonpropagated po- 
tentials, their effect upon the electrically excitable 

" Drugs such as pentobarbital, for example, can alter pro- 
foundly the pathways that produce pyramidal tract activity 
through thalamocortical relays. This is disclosed by changes in 
latency of several msec, when 4 to 10 mg per kg of pentobarbital 
are administered (99, loi, 161 and unpublished work). 



membrane of the same cell depends upon the spatial 
arrangement of these differently excitable structures. 
Assuming as a first approximation that the elec- 
trically excitable membrane everywhere in a cell is 
triggered to discharge a spike by the same level of 
critical depolarization, and that the depolarizing 
p.s.p.'s are everywhere equal in amplitude, the in- 
tensity of excitation of the former by the latter will 
depend upon the distance between the synaptic 
focus and the nearest conductile membrane. The 
more closely the two electrogenic membrane sites 
approximate each other the more intense will be the 
excitation for triggering a spike. The apical dendrites 
of the cerebral cortex are not electrically excitable 
(107) and the p.s.p.'s of the axodendritic synapses 
generated at some distance from electrically ex- 
citable membrane therefore would not be expected 
to be as effective as the axosomatic p.s.p.'s generated 
in close contiguity with electrically excitable mem- 
brane. Thus, the apical dendrites of cortical neurons, 
although they generate intense synaptic activity 
(165) are not as effective in triggering spikes as are 
the depolarizing synaptic loci at the soma (27, loi). 
Spatial considerations may also be applied to the 
effects of hyperpolarizing p.s.p.'s. The latter would 
be most intensely inhibitory if they are interposed 
between sites of excitatory p.s.p.'s and electrically 
excitable membrane. The depolarizing p.s.p., in 
that case, would be diminished not only by elec- 
trotonic averaging between the opposed electro- 
genic actions. The interposed hyperpolarizing site 
would receive more outward current flow than rest- 
ing membrane since its resistance would be lower, 
and the potential gradient steeper. Consequently 
this bypass would result in less current flow from 
the depolarizing synaptic site to the electrically 
excitable but as yet inactive membrane. Thus, the 
loci at which depolarizing and hyperpolarizing 
p.s.p.'s are generated, both relati\e to each other 
and to electrically excitable membrane, must play 
an important role in determining the effectiveness 
of transmission from a given afferent volley. The 
simplifying assumption that all s\naptic sites are 
electrogenically equivalent is probably not justified 
(see below). It is also likely that the conductile 
membrane in different parts of a cell varies with 
respect to its electrical threshold (79) or that it is 
differently electrogenic (69), and these factors may 
reinforce the transmissional inhomogeneity of dif- 
ferent synaptic sites. 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



183 



Physiological Factors Determining 
Transmissional Effectiveness 

a) SYNAPTIC POTENCY AND DRIVE. Just as different 
parts of the same cell exhibit variations with respect 
to electrical threshold so also do different cells in a 
population, that is, the critical firing level may be 
lower for one cell than for another. In that case, an 
afferent volley equally effective in generating p.s.p.'s 
in all the cells may discharge some of these but not 
others. It is unlikely, however, that the synaptic 
potency of a given influx is identical for all cells. 
Even in single multiply-innervated cells, such as the 
eel electroplaque (4), the p.s.p.'s generated over the 
large surface are of different amplitudes and always 
largest at a definite region of the cell surface. Thus, 
the p.s.p.'s generated in a population of motoneurons 
would vary in magnitude depending upon the 
synaptic potency of the afferents to each cell. This 
variation, added to that of the distribution of elec- 
trical thresholds, results in a population spread with 
respect to excitatory effects or synaptic drives. It is 
ob\'ious that the degree to which the given synaptic 
inflow also excites hyperpolarizing p.s.p.'s as well as 
depolarizing, and the relative spatial distributions of 
the two electrogenic activities will affect the mag- 
nitude of the synaptic drive. 



The differences in synaptic drive deduced above 
adequately account for a mass of experimental data. 
The cells in a population of neurons impinged upon 
by a sample from a population of innervating nerve 
fibers will respond with different degrees of depolariz- 
ing p.s.p.'s. Some of the cells will discharge spikes 
which can be recorded directly (e.g. fig. 12) or by 
means of other effects, as for example by their reflex 
activation of muscle in the case of motoneurons. In 
other cells excited by the afferent volley the p.s.p.'s 
alone are generated. 

b) EXCITED AND DISCHARGED ZONES. Thus, an ex- 
citatory volley causing quantitatively different 
amounts of synaptic activity also divides the popula- 
tion of postjunctional cells qualitatively. One group, 
frequently by far the smaller, falls in the discharged 
zone, the other in the excited zone (fig. 26). In this 
distribution the occurrence and influence of hyper- 
polarizing inhibitory p.s.p.'s may also be considerable 
but need not be discussed in detail, except in the 
extreme case when the neural volley generates pre- 
dominantly or entirely hyperpolarizing p.s.p.'s. In 
that case spike electrogenesis would not occur and 
the volley in isolation may produce no overt effects, 
although direct recording from the cells would dis- 



INNERVATED FACE 




EXCITED CELLS 
DISCHARGED CELLS 



LEAD A 



LEAD B 

FIG. 26. Discharged and excited zones in a row of eel electroplaques on maximal stimulation of 
their three different nerve supplies. Cells 6 to 10 were excited by Nerve I as evidenced by their p.s.p.'s 
and long-lasting homosynaptic facilitation, but did not develop spikes to a single testing stimulus. 
Nerve II caused discharge of spikes in cells 9 to 11, but in addition excited cells 6 to 8 and 12 and 
13. Nerve III discharged cells 12 and 13, exciting also cells 10, 11, 14 and 15. The diagrammatic 
representation of the recording leads shows the method that was used to test this population of cells. 
[From .Altamirano et at. (5).] 



1 84 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 1 



close the p.s.p.'s. These, hke the excitatory p.s.p.'s, 
should vary in amplitude in the population of post- 
junctional cells. An overt manifestation of the in- 
hibitory p.s.p.'s would occur if the cells are at the 
same time producing excitatory p.s.p.'s and spikes or 
causing reflex activity in muscles. More or less selec- 
tive afferent activation of inhibition, and its role in 
spinal reflex activity, was demonstrated by Sherring- 
ton (44, 182; cf. 140). Relatively specific descending 
pathways were found by Sechenov (180) in the frog 
and by Magoun and his colleagues (cf. 147) in the 
mammal. 

c) FACILITATION. Study of spinal cord reflexes also 
demonstrated the existence of the excited zone by 
the effects of temporal and spatial facilitation, the 
excited cells being then termed the subliminal fringe. 
Both types of facilitation depend essentially upon 
the properties of summation and sustained response 



of p.s.p.'s described above. However, subsidiary 
effects also participate which will be discussed later. 
The unitary p.s.p.'s are relatively long lasting, in 
the cat central nervous system having a duration of 
about 15 msec. (figs. 12, 27). For that time at least, 
therefore, an excited cell is somewhat depolarized, at 
first to a large degree, but not to that of the critical 
level for discharge, and then to a smaller amount, 
decreasing with time. 

The presence of the e.xcited cells can be tested by 
applying a second volley either through the pathway 
which delivered the first condidoning stimulus 
(homosynaptic testing) or through another inner\-at- 
ing path (hcterosynaptic testing). 

d) HOMOSYNAPTIC FACILITATION. In this casc, there 
will be no second response, neither an electrical 
activity nor a reflex contraction of muscles, if the 
stimulus interval is verv short. Because of absolute 




B 



msec 



50 mV 




10 msec 



^ — ^^^ 



FIG. 27. Temporal facilitation and shortening of synaptic delays in neurons. .-1, B: From a cat 
motoneuron at high and low amplification. Two orthodromic stimuli, neither capable of discharging 
the cell, can evoke a spike by summation of the p.s.p.'s produced by each stimulus. Since the spike 
occurs only when the critical level of depolarization is attained, the summation interval may be 
sharply delineated as shown in this example. [From Brock el al. (24).] C: From a rabbit cervical 
sympathetic neuron. Progressively stronger stimuli to the preganglionic nerve increased the p.s.p. 
of the neuron and evoked its spike earlier as the critical firing level (shown by arrows) was attained 
earlier. [From Eccles (64).] 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



l8n 



% 

zoo 












""■ 300 
Z50 


\ 


\ 










B 


150 


\» 


\ 


^v. 


■~. 




200 
150 


- 


\ 


V 




.^ 














2 


4 


6 


a 


10 


12 Msec ( 


3 


2 


4 


6 


8 


10 


12 Msec. 




FIG. 28. The time courses of facilita- 
tion and direct inhibition (magnitude of 
response as percent of control value) 
tested on monosynaptic reflexes are 
nearly symmetrical. Facilitation (upper 
curves^, A in an extensor and B in a flexor 
muscle. Inhibition (lower curves'), i4 in a 
flexor and B in an extensor muscle. 
[From Lloyd (140).] 



e 10 12Msec 2 4 



12 Msec. 



refractoriness of the presynaptic fibers, no impulses 
arrive at the synapses. At slightly longer intervals, 
relative refractoriness or persistent absolute refrac- 
toriness of the previously discharged postjunctional 
cells causes a depressed testing response, but then an 
interval is reached when the testing response can 
become many times higher than it would have been 
without the preceding conditioning activity. As noted 
above, the facilitation in the simplest cases lasts 
about 15 msec. (fig. 28), decreasing continuously 
from its peak value during this interval. It is likely, 
although this has not as yet been generally estab- 
lished, that the synaptic drive of the testing volley is 
also increased by antecedent activity of the nerve 
fibers. This enhancement may take place in the pre- 
synaptic fibers themselves. For example, invasion of 
the terminal branches by the conductile activity may 
be partial for a single volley and larger for a subse- 
quent. Also, the amount of transmitter released by 
the second activity may be larger. In many junc- 
tional systems, the prolonged stimulation of the pre- 
synaptic nerve at relatively high frequencies for 
some time thereafter increases the effects produced 
by a subsequent single testing stimulus (76, 87, 124, 
135, 139). This phenomenon, post-tetanic (cf 118) 



or postactivating (59) potentiation, may likewise 
depend upon the mechanisms just described. In- 
creased synaptic drive may also involve the post- 
synaptic membrane as, for instance, by a temporary 
change in the excitability of the membrane to the 
transmitter agent. These residual presynaptic and 
postsynaptic effects may alter synaptic drive in either 
direction and act without relation to the residual 
p.s.p. from the first volley. Thus, the homosynaptic 
facilitation which occurs in the eel electroplaque 
(fig. 29) lasts for about i sec, whereas the p.s.p. 
lasts only 2 to 3 msec. (4, 5, 6). 

e) HETEROSYN.'SiPTic FACILITATION. Hetcrosynaptic 
testing eliminates the complications introduced in 
homosynaptic facilitation except the refractoriness 
of the discharged postjunctional cells. The facilita- 
tion now may start at very brief intervals between 
the stimuli. Strictly speaking, however, hetcro- 
synaptic testing involves spatial factors for the 
terminations of one pathway may activate different 
synaptic sites than do those of the other. The facilita- 
tion is therefore likely to take place by electrotonic 
additions of the depolarization produced by the test- 
ing stimulus onto the residual level of general de- 



1 86 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 




ZOO 



MSEC 

FIG. 29. The time course of homosynaptic facilitation in a group of eel electroplaques. The testing 
stimulus alone evoked responses shown by the horizontal lines at the end of the graph in B. At 
various intervals after a conditioning stiinulus, the response to the test stimulus became larger than 
this control value and gradually returned toward it. • : Facilitation without treatment with drug. 
A : 1 2 min. after adding 50 >jg per ml physostigmine the cells became somewhat more excitable, 
the whole curve of facilitation being lifted on the baseline of the larger response to the testing stim- 
ulus in isolation. This effect presumably developed because of the anticholinesterase action of phy- 
sostigmine; it is also produced by prostigmine. The two substances, however, have opposite synaptic 
action, physostigmine being an inactivator of synaptic electrogenesis and prostigmine an excitant, 
n ■ 64 min. later, the physostigmine had depressed synaptic excitability and the whole curve had 
fallen. Expressed in percentile values of the response to the testing stimulus alone in each condition, 
the three curves had essentially the same magnitudes and time courses (.4). [From .\ltamirano 
et al. (6).] 



polarization remaining from the prior stimulus. This 
is, indeed, the condition found experimentally (fig. 
30^) in the electroplaques from the Sachs organ of 
the eel. Both depolarizing p.s.p.'s being short, 
facilitation occurs only during the first 2 msec. In 
cells of the main organ, however, heterosynaptic 
facilitation lasts some 50 to 75 msec. (fig. 3oi?) and 
in this case the effect must be due to alteration of the 
excitability of the synaptic membrane since pre- 
synaptic interactions are ruled out. The different 
behavior of the electroplaques in the two organs is 
probably ascribable to difTerent spatial relations of 
their synapses. If those in the electroplaques of the 
main organ are closely spaced, diffusion of transmit- 
ter from the sites activated b\- the conditioning 
volley might affect the excitability of the synaptic 
loci innervated by the second neural pathway (95). 
The data presented above derive from a particu- 
larly favorable structural configuration, a large 
postsynaptic cell with an extensive responsive mem- 



brane (about 15 mm- in area) diffusely innervated 
by several easily isolated nerve trunks. The experi- 
mental conditions that obtain in nerve cells do not 
usually perinit as clear a delineation between dif- 
ferent spatial interactions. However, in the case of 
cells with long dendrites, as in the cortex, it may be 
expected that interaction between different axo- 
somatic synapses will be greater than that between 
these and the axodendritic. 

f) sp.'Kti.'^l sum.m.ation of converging p.^th\v.\ys. 
Another variety of spatial summation is more fre- 
quently noted in the central nervous system. This is 
the case in which two widely separated neuronal 
complexes eventually converge upon one or more 
common paths. In that final common path the situa- 
tion then reduces to a variant of the case discussed 
above. These convergent types of interaction are 
further complicated in the central nervous system 
by the involvement of inhibitory p.s.p.'s. Spatial 



SYNAPTIC AND EPHAPTIC TRANSMISSION 1 87 




FIG. 30. Heterosynaptic facilitatory actions in eel electroplaques from different electric organs. 
Left: Absence of heterosynaptic facilitation in cells from the Sachs organ. .Activity of one nerve 
evoked the response seen in A. This response -was preceded by that evoked through another nerve in 
B to G. Only when the two stimuli (shock artifacts on the left of the records) were less than i msec, 
apart (_F, G) was there a significant amount of facilitation caused by electrotonic summation of the 
brief p.s.p. [From .-Mtamirano et al. (5).] Right: Heterosynaptic facilitation in cells of the main organ. 
A: The response to stimulating one nerve trunk. B: This nerve trunk is used to deliver the con- 
ditioning stimulus (artifact at the left, upward); the testing stimulus is applied at various intervals 
later to another nerve trunk (artifact down, superimposed traces). Marked facilitation reached a 
peak at 10 to 15 msec, and persisted through the end of the record at 25 msec. C: Nerve 2 was cut, 
and a third nerve trunk was used for the testing stimuli. No facilitation occurred. [From Albe-Fes- 
sard & Chagas (i).] 



inhibition or facilitation may develop particularly 
in the more complex varieties of synaptic organiza- 
tion. The precise effects would depend on the specific 
pathways and electrical responses involved and can- 
not be discussed in this chapter (cf. 99-101, 161). 



186). Thus, they can provide sites at which synaptic 
potentials of both signs may be generated and this 
electrical summation propagated electrotonically to 
act upon an electrically excitable membrane distal 
to the cell body. 



Integrative Utility of Electrical Inexcitability 

The foregoing group of integrative activities de- 
pends essentially upon graded, algebraically sum- 
mative potentials of opposite signs which are made 
available in synaptic transmission by electrical in- 
excitability. In some neurons, relatively large scale 
areas of membrane are not electrically excitable and 
this would appear to aid integrative functions. The 
superficial cortical dendritic surfaces, richly supplied 
with synaptic inflows, are an example of this. The 
synaptic activity that goes on at these dendrites re- 
sult in algebraically summated potentials. Since 
these dendrites are not electrically excitable, the po- 
tentials must be transmitted electrotonically to the 
electrically excitable membrane of the pyramidal 
neurons. In each of these the potential can serve to 
modulate responsiveness to other, more potent 
synaptic inflows. The soma of lobster cardiac gan- 
glion cells also are not electrically excitable (33, 109, 



Synaptic Determinants of Different Types of Reflexes 

In the general context of principles, the precise 
structural and functional complexity of a reflex 
pathway is of little moment. Therefore, the specific 
properties of monosynaptic or multisynaptic reflexes 
need not be dwelt upon since they are finally refer- 
able to the intensity of synaptic drives upon the final 
common path. The analysis of synaptic mechanisms 
in many varieties of reflex response can likewise be 
simplified by merging all interneuronal activities with 
that of the final common path, essentially involving 
a reduction to the monosynaptic case. 

Synaptic organizations involving very strong 
synaptic drive for depolarizing p.s.p.'s will manifest 
themselves by large synchronized efferent electrical 
activity or a twitch-like contraction in response to a 
single afferent volley. The amplitude of the response 
will depend upon the proportion of neurons that lie 
in the discharged zone. The lower the proportion of 



1 88 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



cells with depolarizing p.s.p.'s above the critical 
level, the greater will be the degree of facilitation on 
repetitive stimulation. If the majority of the cells in 
the excited zone develop p.s.p.'s only slightly below 
the level of critical depolarization, a few repetitive 
stimuli will rapidly evoke a maximal response. This 
gives rise to the general class of cPemblee reflexes 
(cf. 44). Augmentation of p.s.p.'s (synaptic facilita- 
tion) discussed earlier (p. 168) also will favor pro- 
duction o{ d' em blee refiexes. 

When the p.s.p.'s of the excited zone are small, 
the responses may recruit very gradually with repeti- 
tive stimuli. Particularly when the synaptic organi- 
zation is complex and the synaptic drives are weak 
will the latency of the response be long. Under these 
conditions it may also be expected that moer fre- 
quent stimuli will shorten the latency markedly and 
increase the rate of growth of the response and per- 
haps its maximum value. In other words, the more 
weakly effective synaptic drives, including multi- 
synaptic pathways, will show a greater frequency- 
dependence. Since the production of hyperpolarizing 
p.s.p.'s also involves excitation of synapses, the de- 
velopment of inhibitory activity will depend similarly 
upon the stimulus parameters. 

Another effect in which the complexits of the 
synaptic organization plays a role is that of after- 
discharge. The involvement of multisynaptic path- 
ways carries the likelihood that additional side 
paths will also be brought into activity and thus 
give rise to a circulating activity (78) or a series of 
delayed reverberations which may cause discharges 
of the final common path long after the initial 
stimulus is ended. This reverberation may take place 
by one-to-one excitation, but it is likely that another 
phenomenon plays an even greater role. This is the 
summation and persistence of p.s.p.'s associated with 
accumulation of a persistent transmitter agent. As 
individual Renshaw cells are capable of persistent 
repetitive discharge by a single stimulus (61), so 
some of the interneurons mediating excitatory 
p.s.p.'s can also remain active for a long time. The 
interplay of excitatory and inhibitory synaptic 
activity may produce complex patterns of waxing and 
waning after discharge. In individual cells this pat- 
terning would be reflected by a greater or lower 
frequency of discharge. Complex interactions of 
excitatory and inhibitory types occur even in the 
relatively simple nuclear structures like that of 
Clarke's column in the spinal cord (104, 115). The 
involvement of a widespread network of neuron 
complexes in after-discharge is indicated also by the 



fact that increasing the strength of the initiating 
stimulus may lead to no increase in the maximal 
amplitude of the reflex respon.se but only in the dura- 
tion of its after-discharge (182). 

Role of Inlnhitioii in Central Nervous System 

The interrelations of depolarizing and hyper- 
polarizing p.s.p.'s in the.se various manifestations, 
in.sofar as they are dependent upon the specific 
organization of synapses, are beyond the scope of 
this chapter, but some general discussion is ap- 
propriate (cf. 1 01, 161). As was described earlier, 
hyperpolarizing p.s.p.'s need attain only relatively 
small amplitudes to produce inhibition. The effect, 
a sudden cutting off of conductile activity, may 
block the synaptic transfer to many systems which 
would normally participate in an activity. The re- 
sults of a given excitatory and inhibitory interaction 
will differ depending upon the site at which an index 
of the effect is obtained. In a specific example let us 
assume that a single cell is acted upon by the synaptic 
interplay. Whether or not it is excited to produce a 
spike will have important consequences for the ac- 
tivity of other downstream neurons for which the 
cell chosen as an example serves as a valve. How- 
ever, when recording from the interior of the cell, 
depolarizing and hyperpolarizing activities may be 
oi)served even in the absence of a spike. Thus, dif- 
ferent criteria apply to activity in different parts of 
a complex pathway. The relations between activity 
in one part and another may even be dimmed or 
may disappear. 

The activity set into motion by a synaptically 
complex pathway thus may be undetected in the 
overt response. For example, a single stimulus to the 
head of the caudate nucleus in the cat giv-es rise to a 
relatively simple, brief electrical response in a re- 
stricted cortical region. Analyses with paired or 
repetitive stimuli disclose (167) that many excitatory 
and inhibitory influences are activated, some for 
long periods of time. It is worth noting that ana- 
tomical data can rarely give information as to the 
presence of such intricate synaptic linkages and, of 
course, cannot distinguish those that are excitatory 
from the inhibitory. 

It is most likely that in its normal functioning the 
central nervous system utilizes inhibitory activity as 
a means for braking excitatory activity which might 
otherwise be unduly prolonged or inclined to rever- 
beration. In that sense, therefore, inhiijitory synaptic 
electrogenesis would aid the precision of ner\ous 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



189 



activity. The remo\al of inhibitory electrogenesis 
either by drugs which specifically blockade hyper- 
polarizing synapses, or by pathological conditions, 
would then remove these brakes upon excitatory 
activity and abnormal function would result. This 
would be apparently caused by 'excitation' although 
its fundamental mechanism would in reality be the 
block of another, opposed type of synaptic activity, 
the inhibitory electrogenesis. The pharmacological 
classification of strychnine as a 'stimulant of the 
central nervous system', already discussed, illustrates 
the basic difference between a descriptive, phe- 
nomenological classification and that ba.sed on 
analysis of its mode of action. 

Phynulogical Effects of Different Proportions oj De- 
polarizing and Hyperpolarizing Postsynaptic Potentials 

Apparently different physiological and pharma- 
cological properties may result from different pro- 
portions of the two kinds of synaptic activity. Thus, 
the electrical activity of the cat cerebral cortex 
differs profoundly from that of the cerebellar, but 
these differences may be accounted for by the rela- 
tively small degree of inhibitory electrogenesis ol the 
cerebellar cortex (161-166). Pharmacological dif- 
ferences, such as the insensitivity of the cerebellar 
cortex to local applications of strychnine, are equally 
ascribable to this quantitative factor. 

However, the response of the cerebellar electro- 
cortical activity to different drugs depends upon 
the mode of exciting that activity (Purpura, Girado & 
Grundfest, in preparation; cf. also 99-101, 163). 
Different cerebellopetal afferents may evoke po- 
tentials of different forms at a single cortical site. 
These potentials are composed of different propor- 
tions of excitatory and inhibitory synaptic activities 
as demonstrated by their different reactions to the 
various specifically acting drugs. 

Synaptic Activity and Electrical Concomitants 

The matters discussed under this heading relate 
physiological activity in the central nervous system 
to the methodology of its study by electrophysiologi- 
cal means. They are also considered by Frank in 
Chapter X of this work. 

a) interpret.ations of changes in .'\mplitudes of 
POSTSYNAPTIC POTENTIALS. Since p.s.p.'s are not 
subject to refractoriness but are capable of summa- 
tion and of being sustained, decrease in amplitudes of 
p.s p.'s cannot be ascribed to their refractoriness or 



'occlusion'. A depolarizing p.s.p. therefore can 
diminish only by virtue of the following factors. 

/) The conductile process of the preceding unit 
is blocked by refractoriness. This is probably a minor 
element since profound alterations in synaptic re- 
sponses occur at frequencies of repetitive activity so 
low that refractoriness of electrically excitable re- 
sponses does not occur. 

2} The transmitter of the presynaptic terminals 
may become exhausted or the receptor of the post- 
synaptic membrane may become altered. The latter 
factor has been discussed in connection with de- 
.sensitization (p. 157). 

^) Stimulation at high frequencies may, however, 
produce fused sustained p.s.p.'s that show little or 
no fluctuation from the steady level (fig. gfi, C). 
This effect probably develops when the synaptic 
membrane is maximally excited by the frequently 
released packets of transmitter agent. The steady 
depolarization (or hyperpolarization) can be re- 
corded in the cerebral cortex (cf. 176, fig. 10). 

4) Simultaneous and countervailing development 
of hyperpolarizing p.s.p.'s may mask the depolariz- 
ing. There is now considerable experimental evidence 
that this factor is most important in the complex 
synaptic organization of the central nervous system 
(165-167). Indeed, the overt electrogenesis ob- 
servable in the cerebral cortex after a single stimulus 
may be only a small part of the total electrogenic 
activity. The major part is not recorded because 
depolarizing and hyperpolarizing p.s.p.'s are simul- 
taneously produced and tend to cancel each other. 

b) interpretation OF electrotonic effects of 
STANDING postsynaptic POTENTIALS. It has been 
frequently assumed that the surface negativity of the 
cerebral cortex caused by dendritic p.s.p.'s produces 
anodal polarization of their cell bodies (cf. i 76, pp. 
56 and 57). This conclusion is drawn from analogy with 
the effects of externally applied currents, a cathode 
on the surface depressing and a surface anode aug- 
menting excitability of the cell bodies. This analogy 
is not valid in the physiological case. Externally 
recorded negativity means that the interior of the 
generating site is depolarized (i.e. more positive than 
the resting potential). Surface negative p.s.p.'s of 
apical dendrites therefore must always act as an ex- 
citatory (cathodal) stimulus for the electrically ex- 
citable membrane of their cells (cf. 97, loi). 

c) synaptic transducer action AND electro- 
genesis. The recorded electrical activity might even 



I go 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 1 



STIMULUS 
Subthreshold Thretshold 




FIG. 31. Ephaptic excitation of squid giant axon. Two nerves 
are arranged as siiown in diagram. I. Contact between nerves 
in sea water. A weak stimulus (/(/O evokes a local response of 
the pre-ephaptic fiber (seen in trace .4). This is not propagated 
to the ephapse and has no effect on the latter (trace B). When 
a pre-ephaptic spike was evoked by a stronger stimulus (.4'), 
the post-ephaptic nerve generated a locaJ response (B'). .^head 
of it is seen the electrotonic pick-up of the pre-ephaptic spike. 
II. Excitability of the axons was increased by removing calcium 
ions from the medium. The weak stimulus still could not evoke 
activity in the post-ephaptic fiber (B), since conductile activity 
was lacking in the pre-ephaptic unit QA). When a stronger 
stimulus evoked a spike (.4') the postephaptic fiber also pro- 
duced a spike (B')- This arose on a step which is the local 
response of the postephaptic fiber (seen in isolation on the 
lowest trace). [From Arvanitaki (10).] 



be absent if ionic processes leading to hyperpolariz- 
ing and depolarizing p.s.p.'s were equally balanced. 
Despite this, however, the transducer actions ini- 
tiated by the excitants of depolarizing and hyper- 
polarizing synaptic membrane would still take place, 
and the ionic transports of the transducer action 
would still occur. Thus, ionic, metabolic and other 
biochemical effects might be produced in the ap- 
parent absence of electrical activity (96, 97). 



EPH.APTIC EXCITATION 

Electrical Modes of Transmission 

In the course of efforts to validate the theory 
of electrical transmission many attempts were made 



to confirm Kiihne's dictum that "a nerve only throws 
a mu.scle into contraction by means of its currents 
of action." In 1882, Hering (i 10) found that a nerve 
\-olley initiated in one distal branch of the frog 
sciatic nerve and coursing centrally in the whole 
nerve trunk could set up activity in another branch 
when the impulses arrived at the centrally transected 
stump of the nerve. The current flow generated by 
the active fibers must have stimulated the previously 
inactive fibers. The effect has been confirmed many 
times (cf. 149) but nowhere more clearly than in a 
preparation involving two squid giant axons (lo). 
It must be emphasized that specially favorable ex- 
perimental conditions are required to produce this 
■ model' of transmission which is termed an ' ephap.se' 
(false synapse). In the squid giant fiber (fig. 31) the 
electrical excitability of the ephaptic region is 
heightened by depriving the medium of calcium. 
The extrinsic current of the spike in the pre-ephaptic 
terminal is then capable of acting as a sufficiently 
strong electrical stimulus to evoke a postephaptic 
spike. As a weaker stimulus, it can elicit a graded 
local response. In more complex geometrical con- 
ditions between active and inactive cells, the direc- 
tions of the extrinsic or field currents may produce 
hyperpolarizations as well as depolarizations (figs. 
I, 32). The activity travelling in one fiber generates 
extrinsic current fields in contiguous parallel fibers 
which have a triphasic sequence (126) that suc- 
cessively produces hyperpolarization and depressed 
excitability, then depolarization and heightened 
excitability, followed again by hyperpolarization 
and depression (fig. 32). 

A weakness of ephaptic transmission as a model 
of synaptic activity lies in the fact that basically it 
does not offer a mechanism for polarized transmis- 
sion. Thus, in figure 31 the ephaptic excitation might 
very well have taken the opposite direction, from 
nerve B to nerve A. Special geometric properties 
were invoked by du Bois-Reymond and by Eccles 
(figs. I, 32), and tlie latter also introduced the special 
electrophysiological rectifying effects of anodal and 
cathodal currents (fig. 32). These conditions might 
account for polarized transmission with an electrical 
mechanism; and, as will be described below, a high 
degree of rectification recently discovered in one kind 
of junction (83) does polarize conduction. However, 
the crucial distinction is whether or not current 
flow in a presynaptic terminal, or current flow im- 
posed through the synaptic junction, can excite the 
activity of the latter. The an.swer, illustrated in this 
chapter with a number of examples (e.g. figs. 6, 19), 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



191 



120- 



110- S 



100 



90- 



80- 



Action potential in 
fibre I 




Excitability chonge in 
fibre IE 




>A,X»-TC 



Ct1"<AAt 



i':i.|:>iaJ-t 




msec. * — * — 

FIG. 32. Excitability changes caused by field currents. I'pper lejt: A spike was produced by a stim- 
ulus to one of a pair of crab nerve fibers as in diagram upper right. The electrical excitability of the 
second fiber is shown (lower left) in relation to the time at which the spike passed the testing region. 
In the interval before the spike had reached that site, the excitability of the fiber was depressed. 
During the time that activity resided at the tested level, the excitability was augmented. This was 
followed by a second depressed phase as the activity propagated out of the tested site. [From Katz & 
Schmitt (126).] Right: Diagrams of the anodal, cathodal and anodal polarizing sequence generated 
in the inactive fiber by the spike in an adjoining fiber ilop') and of different field current conditions 
produced by different geometrical arrangements (bottom). [From Eccles (57).] 



seems to be clear. The current flowing across an 
active presynaptic terminal and across the post- 
synaptic membrane appears to be far too small to 
excite the postsynaptic cell. Furthermore, the proc- 
esses associated with synaptic activity cannot be 
initiated by very strong applied currents. 

Role of Field Currents in Central Nervous System 

The activity of masses of cells or fibers in the 
central nervous system is particularly conducive to 
development of field currents within the volume of 
this structure (15). This fact suggested (88, 90) that 
field effects might play a role in determining the 
peculiarities of central nervous properties. The 
hypothesis appeared to have been confirmed by anti- 
dromic stimulations of motoneurons which altered 
the responses of contiguous motoneurons to a testing 
afferent volley (170). That conclusion, however, is 
invalidated by the subsequent finding (171; cf. 60) 
that the antidromic stimuli evoked synaptic activity 
within the spinal cord by means of the recurrent 
collaterals of the motoneurons. 

Although field currents undoubtedly play some 
role (cf. 106), their wide significance must now be 



questioned in the light of the evidence that synaptic 
transmission is not effected by electrical stimuli. 
Changes in membrane potential produced in one 
cell by activity of contiguous elements appear to be 
small (33, 59, 125), although effects may be revealed 
by tests on electrically excitable membrane (106, 
126, 185). However, the effects exerted electro- 
chemically on p.s.p.'s (as described in the section in 
this chapter on the nature of postsynaptic potentials) 
are probably insignificant. Thus, electrical ine.x- 
citability renders the transmissional process insensi- 
tive to fields of current in the central nervous system 
(93). Teleologically considered, this is probably an 
advantage. The fields must shift from moment to 
moment as the loci of activity shift in the cellular 
mass of the volume conductor. The effects of these 
fields must therefore be highly unspecific, now pro- 
ducing increase, now depression of electrical ex- 
citability, actions that probably would disturb the 
precision of organized orderly synaptic transfer. 
Thus, electrical inexcitability of synaptic membrane 
removes a major hazard, that irregular effects of 
electric fields might disrupt the patterned activity 
of the central nervous system. 



192 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



Dorsal Root Reflex 

Acting upon electrically excitable components, 
however, field currents might still affect central 
nervous functioning. For example, small depolariza- 
tion of a cell by field current might facilitate its 
discharge by an otherwise subliminal depolarizing 
p.s.p. Likewise, presynaptic terminals close to an 
active synaptic focus might be subjected to con- 
siderable potential change (51), an action which 
may account for the dorsal root reflex (188). This 
prolonged centrifugal discharge of dorsal root fibers 
is evoked with a latency of some milliseconds after 
the same or other dorsal root fibers cany a volley 
centripetally. The dorsal root reflex is enchanced 
by low temperatures as is the motor root reflex (29, 
89). The latency, temperature effect and prolonged 
discharge of the dorsal root reflex indicate that it is 
produced by a synaptic activity in the spinal cord, 
and yet there is no histological evidence of synaptic 
inflows to the dorsal root collaterals. In the absence 
of the latter, ephaptic excitation may be invoked, 
but will involve synaptic pathways also. This could 
result from the field effects generated in the dorsal 
root terminals by the activity of some interneuronal 
pools. The activity of these cells, being evoked by 
synaptic transfer, would account for the apparent 
synaptic properties of the dorsal root reflex, but its 
final development would be by ephaptic excitation. 

Ephaptic Transmission in Annelid and 
Crustacean Nerve Cords 

In many species of these invertebrates there occur 
junctions (septa) between anatomically distinci 
elements, the segments of the septate giant axons. 
Across the septa considerable electrotonic current 
flow can take place and ephaptic electrical transmis- 
sion is then possible (125; Kao, C. Y. & H. Grund- 
fest, manuscript in preparation). The junctional 
membranes of these functional ephapses must there- 
fore be fundamentally different from those of 
synapses, across which only insignificant electrotonic 
current flow occurs. However, the anatomical data 
to account for this difference are still unsatisfactory. 
The transverse sheaths which separate abutting seg- 
ments of the septate giant axons appear to be identi- 
cal with the sheaths that invest the axis cylinders 
(cf. 1 25). On the other hand, the junction between 
the medial giant axon and the motor giant fiber 
of crayfish seems to be formed by processes from the 
postjunctional motor nerve which penetrate the 
Schwann sheaths to make intimate contact with the 



cell membrane of the prejunctional fibers Ci74)- The 
junctions between two motor giant axons are also 

similar. 

a) unpol.\rized EPH.'SiPTic JUNCTIONS Thcsc havc 
been studied with intracellular recordings in the 
.septate giant axons of earthworm (125) and cray- 
fish (Kao, C. Y. & H. Grundfest, manuscript in 
preparation). The septa, sometimes called 'un- 
polarized macros\napses' (cf. 30, 125), appear to be 
merely the boundaries demarcating the multiple 
origins of the .septate giant axons from a number of 
segments of the animal. Activity in one segment of 
the axon causes electrotonic potentials in the neigh- 
boring segments large enough to excite the latter. 
Thus, transmission is by local circuit excitation, es- 
sentially as in other axons. As in the latter, the 
ephaptic transmission of the septate axons is un- 
polarized, capable of propagating an impulse in 
either direction. 

b) pol.-^rized eph.-vptig transmission. One system 
recently described (83), the junction between cord 
giant fibers and efferent motor giant axons of cray- 
fish, may be classified in this category. Current flow- 
ing outward from the depolarized prefiber can enter 
the junctional membrane of the postfiber, causing 
large depolarization in the latter (fig. 33.-1) and its 
ephaptic excitation. However, when the postfiber is 
depolarized (fig. 335) the electrotonic effects in the 
prefiber are small. Likewise when the prefiber is 
hyperpolarized current flow in the postfiber is hind- 
ered (fig. 33.4), while hyperpolarizing the postfiber 
causes large electrotonic changes in the prefiber 
(fig. 33^). The junctions thus exhibit rectification, 
with conductance in one direction (that tending to 
depolarize the postfiber) about 20 times greater than 
in the opposite direction. Thus, in the case of the 
motor giant fiber ephapse, the low electrical resist- 
ance in one direction and high resistance in the other 
makes for polarized ephaptic transmis.sion. 

Since the junction meets the criteria of anatomical 
discontinuity and transmissional polarization, it 
fits the definition of synapse extant since Ramon y 
Cajal and Sherrington. However, though it may be 
called an 'electrically excitable synapse' (83), it 
probably differs profoundly from the electrically 
inexcitable synapses discussed in this chapter. The 
distinction between ephaptic junctions which have 
low electrical resistance and synapses which have 
high resistance helps to make the classification more 
precise. Thus, experiments similar to those shown in 



SYNAPTIC AND EPHAPTIC TRANSMISSION 



'93 



Vtt 



MV 




hypcrpolorizotion 



8 



5H 



'9^ 



^'post 



i^ 



hypcrpolarlzotion 



depolarizotion 



-5 



'pre 



•-lO 
mV 



FIG. 33. Rectification at the junction be- 
tween a cord giant fiber and a motor giant 
axon in crayfish results in polarized ephaptic 
transmission. A: Current was allowed to flow 
through a microelectrode in the prejunctional 
cord giant axon. The changes in the membrane 
of the same fiber were recorded with another 
microelectrode and are shown on the abscissa. 
The ordinate indicates the membrane voltage 
recorded at the same time with a micro- 
electrode in the postjunctional fiber. When 
the prefiber was depolarized, a steeply rising 
depolarizing change also took place in the 
postfiber. As an extrinsic local circuit change 
was produced by a spike in the prefiber, it 
would lead to an electrically excited response 
of the postfiber. When the prefiber is hyper- 
polarized (left side of .-1), only small changes 
in potential develop in the postfiber. The ratio 
of current flowing in the two directions is about 
20:1. B: In this experiment current was 
applied to the postfiber, the abscissa shows the 
change in membrane potential of this fiber 
and the ordinate the change in membrane 
potential of the prefiber. When the postfiber is 
hyperpolarized, there is a considerable current 
How into it from the prefiber, causing some 
clectrotonic hypcrpolarization of the latter. 
When the postfiber is depolarized, little 
current flows into the prefiber and it there- 
fore cannot be stimulated by a spike in the 
postfiber. Electrical excitation across the 
junction is thus transmitted only from the 
pre- to the postfiber. [From Furshpan & 
Potter (83).] 



figure 33 were done on the squid giant axon synapse 
by Tasaki. He "could not detect any recognizable 
spread of clectrotonic effects across the synapse in 
either direction" (personal communication). It is 
likely that pharmacological data and various other 
criteria of the constellations listed in table i will 
distinguish the two types of transmission systems 
further. 

Several properties of the polarized ephaptic junc- 
tion may be deduced from the available data and 
from general considerations. Rectification is ex- 
hibited by the membranes of many, though not all, 
cells, although not to the same large degree (cf. 
Tasaki, Chapter III). Where found, it is manifested 
by a higher membrane resistance to inward current 
than to outward flow. In the present case two mem- 
branes are involved and, if both are rectifiers, then 
they must each act in opposite polarity to the other. 
On the other hand, only one of the two membranes 
need show rectification and this situation is the more 



probable. It also seems most likely that this property 
resides in the surface of the prefiber for in that case 
the membrane would permit outward current flow 
and restrict inward as in other cells. The membrane 
of the postfiber would then need have no rectifier 
properties but would resemble that of the septa in 
its low nondirectional resistivity. 

As may be seen from figure 34, current probably 
flows outward across the prejunctional membrane 
during the ephaptic transmissional process whereas 
in the rest of the active region the membrane current 
is inward. Furthermore, excitation of the postfiber 
must occur at membrane sites where the local circuit 
current flows outward, not at the ephaptic region 
where it flows inward. Therefore, neither junctional 
membrane of this polarized ephapse takes part in 
the active responses of the junction. Like the mem- 
branes at the septa they therefore need not be ex- 
citable. 



194 



HANDBOOK OF PH'lSKJLOGV 



NEUROPHYSIOLOGV I 



Intrinsic 



\ 



Local Circuits 
-> 1 I ^ 



Transjunctional 




Ephaptic Junction 

FIG. 34. Diagiam showing the current flows that probably 
take place at a polarized ephaptic junction. In the prejunc- 
tional fiber membrane current flow is inward in the region of 
activity. Longitudinal current flow takes place behind this 
region as part of the intrinsic local circuit within this fiber. 
Current flows outward through the membrane recovering from 
previous activity. Outward current also flows in the prejunc- 
tional membrane of the ephapse. This enters the postcphaptic 
cell at its junctional membrane and flows out through adjacent 
regions of membrane, exciting the latter. Note the profound 
difference between the current flows postulated for ephaptic 
transmission in this diagram and the hypothetical situation al 
synaptic junctions shown in fig. i . 



Evolutiormry Aspects of Ephaplii Transmissum 

In their transverse divisions the septate axons bear 
the sign of their segmental origin. The processes of a 
number of neurons at a nerve cord segment fuse to 
produce a short length of giant axon. End-to-end 
apposition of the segmental fibers then forms a long 
axonal pathway. To the extent that the septa dis- 
appear or that their resistance is low the segmented 
axons approach the nonsegmented giant axons in 
efliciency as through conduction pathways, excited 
by local circuit action. 

The septate axons, however, combine with through 
conduction, another feature which is absent in the 
nonsegmented giant fibers (Kao, C. Y. & H. Grund- 
fest, manuscript in preparation). They make elaborate 
local synaptic connections, both efferent and afferent, 
with other fibers of the nerve cord. Although the 
anatomy of these connections is not as yet clear, the 



synaptic properties of the septate axons probably 
derive from their segmental origin of the fibers. The 
septate giant axons therefore play a much greater 
role in the integrative activity of the nervous system 
than can the nonseptate axon which lack these synap- 
tic connections (103, 125). 

On the basis of the interpretation given in the pre- 
vious paragraphs, the polarized, electrically excitable 
ephaptic junction may be derived from the septal 
junctions by the addition of rectifier property to 
one of the junctional membranes. Two other features 
further strengthen the resemblance between septate 
and motor giant fibers. The two motor axons of a 
segment make unpolarized connections with each 
other. In this case, too, electron microscopy has not 
as yet revealed essential details (cf. i 74). Also, like 
the septate axons, the motor giant fiber combines 
'chemically mediated synapses' with an ephaptic 
junction (83). The former presumably are electrically 
inexcitable. 

Thus, it appears likely that motor giant fibers of 
the crayfish bear a close functional similarity to the 
septate axons but with a significant modification away 
from the latter. It remains to be seen whether ephap- 
tic polarized transmission made possible by rectifica- 
tion is a fairly common evolutionary variant. Another 
interesting correlation, whether or not this transmis- 
sion scheme developed only in those animals that 
have septate unpolarized ephapses, might give fur- 
ther clues to their evolutionary origin. 

Qjiasiartificial Synapses 

The excitation of giant nerve fillers in annelid 
nerve cords by activity in other giant axons is well 
documented (31) and may be an ephaptic phe- 
nomenon In Protida the sites of transfer vary from one 
occasion to another and have been termed quasi- 
artificial synapses. These systems have not yet been 
studied with intracellular recording. The latter could 
help to determine whether the transmission is ephap- 
tic or whether it is associated with complex synaptic 
phenomena such as have been found in earthworms 
(125)- 



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1946. 

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139. >947- 

Lettvin, J. Y., W. S. McCuLLOCH and W'. Pitts. Am. J. 

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641, 1956. 



CHAPTER VI 



Skeletal neuromuscular transmission 



PAULFATT I Biophysics Department, University College, London, England 



CHAPTER CONTENTS 

Morphology 

Local Electrical Response 

Activity of the Nerve Terminals 

Properties of the Junctional Receptor 

Conclusion : Mechanism of Transmission 



THE EXISTENCE OF A REGION between motor nerve and 
voluntary muscle which has special properties 
emerged from experiments on the action of the South 
American Indian arrow poison, curare, performed lay 
Claude Bernard about 1850 (2). Bernard's aim in the 
first place was to show that muscle was excitable inde- 
pendently of its nerve supply. Having; pre\iously 
paralyzed a frog with curare, he isolated a nerve- 
muscle preparation and showed that, while an elec- 
trical stimulus applied to the nerve was ineffective, 
a contraction resulted if it were applied directly to 
the muscle. Inferring that curare interfered with the 
functioning of the nerve but not of the muscle, he 
carried the investigation a step further by preparing a 
frog with a ligature which interrupted the blood 
supply to the hind legs but not the nervous connec- 
tions. When curare was introduced above the liga- 
ture, a paralysis developed which affected only the 
anterior part of the body. Most significant was the 
observation that pinching the skin above the ligature 
did not elicit movement in that region but caused the 
normal reflex thrust of the hind legs. It was concluded 
from this that curare did not cause a loss of sensation, 
and its effect was therefore ascribed to a poisoning of 
the motor nerve, for, as was already seen, in the 
presence of curare the muscle could still be excited 
directly. But since curare apparently did not affect 
the motor nerve in its more central course from the 



spinal cord to the level of the ligature either, it wa 
maintained that the poison acted on the motor nerve 
only in its most peripheral part, where contact was 
made with the muscle. 

Following this penetrating study, investigations 
were carried out over a number of years by other 
workers into the method of action of substances that 
affect nerve-muscle transmission more or less specif- 
ically. Besides curare, one of the chief of these was 
nicotine. When a small amount of this drug was 
injected into an animal or added to the solution 
bathing an isolated muscle, a contraction occurred 
which was abolished by curare at the same time as 
was the contraction produced by nerve stimulation. 
It was further found that chronic denervation did not 
eliminate the capacity of the muscle for responding 
to nicotine, which was still antagonized by curare, 
although the nerve terminals underwent severe 
deterioration (34, 46, 57, 59). From this it was con- 
cluded that the site of action of curare, as well as 
nicotine, was not in the nerve endings, as had pre- 
viously been supposed, but in the muscle fiber. 

A quantitative investigation of the effects of these 
substances was made by Langley about 1910 (58, 
60, 61). By the application of small droplets of nicotine 
solution along a mu.scle fiber, he found that nicotine 
in low concentration initiated a contraction only when 
applied in the region of the nerve endings. A concen- 
tration one thousand times greater than the minimum 
effective dose was required to produce a contraction 
elsewhere along the muscle fiber. Furthermore, 
curare interfered with the action of nicotine in low 
concentration but had no effect on the contraction 
produced by the high concentration that did not act 
exclusively in the innervated region. The manner in 
which curare and nicotine acted was inferred from the 
observation that increasing concentrations of curare 



'99 



200 



HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY 1 



were able to antagonize increasing concentrations of 
nicotine over a wide range of such concentrations, 
and that the effects of the two substances were to some 
extent reversible, the same result being achieved 
irrespective of the order of their application. This led 
to the suggestion that nicotine and curare competed 
with each other in forming a loose combination with 
a 'receptor substance,' which was thought to occur in 
the muscle fiber immediately around the nerve 
endings where it could be acted upon h\ the nerve 
impulse. Nicotine or the nerve impulse when acting 
on this receptor would lead to a contraction, while 
its combination with curare would prevent either of 
them being effective. 

In 1936 the concept of a distinctive chemical process 
in neuromuscular transmission was given a secure 
foundation by the work of Dale and his followers. 
They succeeded in showing that a nerve impulse on 
reaching the terminals in a muscle caused the release 
of a pharmacologically active substance (iS). On the 
repetitive stimulation of the motor nerve fibers, to the 
exclusion of other types of nerve fibers, a substance 
appeared in the fluid perfusing the muscle that was 
capable of causing a contraction of muscle from the 
leech and a fall in arterial pressure of the cat. From 
the relative effectiveness of the substance on these two 
test preparations and the modification in their 
response produced by drugs, as well as from the chemi- 
cal stability of the substance under various condi- 
tions, it was concluded to be acetylcholine, the 
pharmacological action of which was already known. 
Its release was found to be undiminished when trans- 
mission was abolished by curare. Further experi- 
ments showed that the rapid injection of acetylcholine 
into a muscle by its blood vessels caused the excitation 
of muscle fibers and a contraction (7, 8, 9). This 
occurred in the chronically denervated muscle as 
well as in the normal one, and, as in the case of 
nicotine, this excitatory action could be abolished by 
curare. The effect of physostigmine was also ex- 
amined. It was found to prolong and intensify the 
response to injected acetylcholine and to cause repeti- 
tive muscle discharges to a single nerve impulse. 
From earlier studies it was known that physostigmine 
has the specific action of inhibiting the enzyme that 
destroys acetylcholine. 

All these findings are compatible with the chemical 
theory of transmission, according to which transmis- 
sion is accomplished by the nerve impulse causing the 
release of a small quantity of acetylcholine from the 
nerve endings. This substance combines with a special 
receptor substance in the junctional region of the 



muscle fiber and, by so doing, alters the properties 
of the fiber in such a way as to lead to excitation and 
contraction. This mediation of transmission by a 
specific chemical is fundamentally different from the 
process occurring when an impulse is conducted along 
a continuous structure, in which case an essential 
factor for the spread of excitation is a flow of electric 
current between adjacent parts. An alternative 
explanation of neuromuscular transmission is ex- 
pressed in the electrical theory, according to which 
transmission is affected by the action currents gen- 
erated by the impulse in the prejunctional nerve 
terminals passing through the adjacent muscle fiber 
in the appropriate direction and in sufficient magni- 
tude to cause excitation. This theory was formulated 
when the electrical events associated with the con- 
ducted impulse were first studied, and the attempt 
was made to account for both processes by a common 
mechanism. The selective sensitivity of transmission to 
various treatments was ascriljed to secondary effects, 
in particular to the alteration of the electrical ex- 
citability of the postjunctional structure. 

The results of experiments in which chemicals are 
involved, either the collection of acetylcholine after 
nerve stimulation or the application of various 
chemicals to evoke or modify the response of the 
postjunctional structure, are consistent with the 
chemical theory. A decisive result which excludes the 
possibility of electrical transmission comes from the 
study of the alteration of properties of the post- 
junctional region during transmission. It is found that 
the characteristic alteration responsible for excitation 
of the muscle fiber cannot be brought about by a 
current generated externally to the fiber. On the other 
hand an alteration of precisely this type is produced 
by the application of acetylcholine to the junctional 
region of the muscle fiber. Accepting the correctness 
of the chemical theory of transmission, one is able to 
give an integrated account of a wide range of experi- 
mental observation, distinguishing between those 
events which occur prejunctionally and involve the 
release of acetylcholine, and those which occur post- 
junctionally and involve the reaction of acetylcholine 
with the receptor and the resultant change in the 
properties of the muscle fiber membrane. 



MORPHOLOGY 

The detailed morphological description which fol- 
lows applies to junctions on skeletal muscle in verte- 
brates where the normal response to a single nerve 



SKELETAL NEUROMUSCULAR TRANSMISSION 



201 



impulse is a propagated action potential and a twitch. 
These are the junctions of which both the morphology 
and physiology have been most intensively studied. 
There are other junctions, in the amphibian at least, 
where the normal mechanical response of the muscle 
fibers is a slow tonic contraction which can only be 
elicited in appreciable tension by a train of nerve 
impulses (53, 55). These fibers are innervated by a 
special class of small diameter nerve fibers which form 
numerous, widely distributed terminations of the 
en grappe type on them. 

The twitch muscle fibers are innervated by coarse 
motor nerve fibers. On issuing from the central 
nervous system, each nerve fiber branches repeatedly 
both before and after reaching the muscle it supplies. 
By this branching the nerve fiber forms junctions on 
many muscle fibers, the number varying greatly for 
mu.scles in different parts of the body in a given 
animal. Conversely, muscle fibers have been found 
to be supplied each with a few nerve endings at 
widely separated positions along their length (47, 50). 
These multiple junctions are in some cases made by 
separate nerve fibers and in others bv branches of a 
single fiber. The variations in the distribution of nerve 
fibers to muscle fibers in different preparations and 
their probable relation to differences in function have 
been discussed by Tiegs (73). 

In the morphology of the single junction, the pat- 
tern made by the nerve fiber in terminating also 
shows marked differences from species to species and 
from muscle to muscle. This field was early thoroughly 
explored Ijy Kiihne (56). Confining consideration to 
the more familiar objects of investigation, he drew a 
distinction between the plate type of ending in the 
mammal and reptile and the bush type in the frog. In 
both types the nerve comes into contact with the 
muscle fiber immediately after losing its myelin 
sheath and branches repeatedly on its surface to form 
the terminal apparatus. In the former type, the 
extent of this apparatus is limited to a roughly circular 
space (the endplate) which has a diameter of 25 to 
70 /x. Viewed in a section at right angles to the muscle 
fiber surface this region is marked by a rounded 
eminence. Within the confines of the endplate the 
terminal branches cover a large fraction of the in- 
cluded muscle fiber surface. In the case of the other 
type of ending (the endbush) the nerve terminal 
branches range over a much wider area. The terminal 
apparatus here consists mainly of several large, 
straight branches 100 to 300 /j. in length, running 
parallel to the axis of the muscle fiber and connected 



into a continuous system by shorter lengths at right 
angles. 

As a result of careful cytological examination, it is 
recognized that there are three sharply defined com- 
ponents of different cellular origin at the junction 
(14). The first of these is the terminal apparatus of the 
nerve. The second is the specialized region of muscle 
fiber surface contacted by the nerve endings. A char- 
acteristic of this region is an increased density of 
muscle nuclei (fundamental nuclei of the junction), 
the presence of which is suggestive of a higher degree 
of synthetic activity here than elsewhere in the muscle 
fiber. The third component is a layer of neuroglia 
which in this position is referred to as the teloglia 
and which appear to be continuous with the -Schwann 
cell envelope of the myelinated fiber. It contributes 
about half the nuclei seen in the junctional region 
(the sole nuclei), the remainder being the fundamental 
nuclei in the muscle. It is dispersed over the entire 
endplate where it forms the rounded eminence and 
accompanies the terminal nerve filaments along their 
extended course in the endbush. In spite of the gross 
differences that exist between the two types of ending, 
the detailed relationships between these three cellular 
components are fundamentally the same. From the use 
of cytological and histochemical staining methods it 
appears that the nerve terminal branches lie sunk in 
grooves in the muscle fiber surface (14, 16). Only a 
small part of the circumference of the nerve is not in 
close proximity to the surface of the muscle lining the 
groove. The sides of the groove appear to be marked 
with a set of parallel lines 0.3 to i m apart which are 
oriented more or less normal to the axis of the groove 
and extend a short distance into the muscle beyond 
the clearly defined edges of the nerve cylinder. In the 
case of the endbush, where there are long stretches of 
unbranched nerve fiber, the ruling is highly regular, 
the lines running from one edge of the groove to the 
other without deviating from this geometrical rela- 
tion. In the endplate where the nerve filaments 
usually extend for no more than a few diameters 
before terminating or branching, the arrangement 
of the lines is less regular, adjacent lines frequently 
fuse with one another, while their spacing is main- 
tained relatively constant. Examination of the junc- 
tion with the electron microscope reveals regularly 
spaced narrow infoldings of the membrane of the 
muscle fiber lining the groove (71). These fine junc- 
tional folds very probably correspond to the lines 
seen under the light microscope. Figures i and 2 show 
the relation between nerve and muscle over a wide 
range of magnification. The width of the folds is 



202 HANDBOOK OF PHYSIOLOGY ^ NELfROPHYSIOLOG\' I 



Wfff^ 



5m 








FIG. 1. Surface view of neuromuscular junction of lizarcl stained with Janus green. The only 
parts to have taken up the stain are the regions of muscle bordering the ner\e terminals (the sub- 
neural apparatus) and a short piece of nerve at the termination of the myelin. The final part of 
the myelinated nerve fiber appears in the extreme left of the upper picture. In the lower picture a 
portion of the junction is shown at higher magnification revealing the lines in the subneural appara- 
tus, which are oriented at right angles to the edge of the nerve and which are uniformly spaced 
about 0.4 M apart. [From Couteau.x (15).] 

FIG. 2. Electronmicrograph of lizard neuromuscular junction. Two nerve terminal branches are 
seen in the left side of the main picture with the muscle to the right. The dark oval bodies in the 
nerve and muscle are mitochondria. The surface of the muscle at the junction is thrown into a 
series of folds, which correspond in their repetition interval and depth to the lines in hg. i . From 
the appearance where the surface membrane of the nerve can be clearly seen, it is established that 
it does not enter the folds. The inset gives an enlarged view of the situation at the junction. The 
surface membranes of nerve and muscle probably correspond to the two dense lines separated by 
about 0.07 fi. [From Robertson (71)] 



about 0.05 /i and their depth about 0.5 ^l. This in- 
folding considerably increases the area of postjunc- 
tional membrane which may have an important 
bearing on the magnitude of the alteration produced 
in the junctional region during transmission. The 
teloglia does not occur within the grooves but appears 
to remain in contact with the exposed part of the nerve 
cylinder. This suggests that it plays no direct role in 
the transmission process. 



LOCAL ELECTRICAL RESPONSE 

The study of neuromuscular transmission received 
a great impetus with the application of electrical 
recording techniques to the junctional region. It 
was observed by a number of workers that after a 
muscle had been treated with just sufficient curare to 



prevent contraction from nerve stimulation, there 
still occurred an electrical change in the muscle, 
though this was different from the action potential 
type of response (13, 30, 33, 43, 45, 72). The response 
was not propagated, being recorded in monophasic 
form between different positions along the muscle. In 
the sartorius muscle of the frog, with one electrode 
kept on the nerve-free pelvic end and the other moved 
from place to place, the magnitude of the recorded 
potential change was found to be correlated with the 
density of nerve endings under the moving electrode. 
The potential change arising at a focus of nerve 
endings (recorded with respect to a distant nerve-free 
point on the muscle) consists of a transient negative 
deflection having a relatively brief rising phase and a 
slower return, the later part of which follows an 
approximately exponential time course. This response 
has been generally referred to as the endplate po- 



SKELETAL NEUROMUSCULAR TRANSMISSION 



203 



tential, noivvithstanding that in the amphibian 
muscle, where it has been studied most, the nerve 
ending is not of the morphological form described as 
an endplate. 

On increasing the concentration of curare in the 
fluid bathing the muscle, the amplitude of the response 
is reduced. When, on the other hand, the concentra- 
tion is decreased from that required to block transmis- 
sion, action potentials in individual muscle fibers 
appear as more rapid and complex deflections super- 
imposed on the endplate potential. With further 
reduction of curare, the action potential component 
increases and obscures the endplate potential. The 
endplate potential was early inferred to be developed 
across the surface membrane of the muscle fiber, 
although confined to its junctional region, because of 
the similarity of this potential with that which could 
be evoked by a brief pulse of current applied any- 
where along the muscle. More compelling evidence 
came from the study of the interaction of the junc- 
tional response and the muscle action potential, the 
latter elicited by direct stimulation and propagated 
into the junctional region. It was found by this 
method that the action potential and the endplate 
potential did not sum with each other, and that the 
action potential was capable of aboli.shing the later 
part of the endplate potential when timed to coincide 
with its summit. 

The most accurate basis for an analysis of the 
potential changes in the muscle fiber to determine the 
manner of their generation is the results from intra- 
cellular recording (40). This involves inserting a very 
fine electrode through the surface membrane of 
individual muscle fibers and recording potentials 
between it and another electrode in the surrounding 
fluid. Intracellular recording not only makes more 
accurate measurements of the electrical response 
possible but also greatly simplifies its interpretation. 
After minor corrections for extracellular gradients of 
potential when current is flowing, the potentials 
observed by this method are those obtaining across the 
surface membrane of the muscle fiber at the position 
of insertion of the electrode. The frog muscle fiber is 
found to have a resting membrane potential of 
about 90 mv (inside negative with respect to outside), 
which is the same in the junctional region as elsewhere 
along the fiber. The addition of curare to the solution 
bathing the muscle in a concentration sufiicient to 
block transmission has no effect on this resting po- 
tential. With the intracellular electrode situated in 
the junctional region of the fiber, an endplate po- 
tential is recorded in response to nerve stimulation. 



It appears as a transient positive deflection, i.e. as a 
transient reduction of membrane potential from its 
resting level. Its amplitude varies from fiber to fiber 
and depends upon the concentration of curare. In a 
frog sartorius muscle, critically curarized to abolish 
contraction, different fibers have been found to dis- 
play endplate potentials ranging from i mv to more 
than 20 mv. The response would be expected under 
these conditions to range in size up to the threshold 
depolarization for initiating an action potential 
which would be about 40 mv. Immediatelv at the 
junction the endplate potential has a rising phase 
lasting 1.5 msec. Following the attainment of the 
summit, the potential declines to one half in another 
2 msec. The rate of fractional decay decreases beyond 
this, the time required to fall from one half to one 
quarter being about 5 m.sec. A potential change can be 
detected at points on the fiber up to a few millimeters 
distant from the nerve ending, becoming progressively 
more attenuated and slowed with increasing distance 

(fig- 3)- 

This potential wave has been analyzed to determine 
the movement of charge underlying it. The amplitude 
of the potential at various instants is plotted against 
distance along the fiber. Assuming that the membrane 
capacity remains constant during the response, the 
curves thus formed indicate the spatial distribution of 
charge displaced from the membrane capacity (rela- 
tive to its initial condition of charge). The area 
beneath each curve is a measure of the total charge 
displaced at the given instant. The plot of these areas 
against time shows that the charge is built up to a 
maximum in about 2 msec, and after this it decays 
exponentially with a time constant of about 25 msec. 
A determination of the passive electrical characteris- 
tics of the muscle fiber shows that this latter value 
corresponds to the electric time constant of the mem- 
brane. This result is consistent with the idea that 
there is a brief phase of transmitter action, confined to 
about the initial 2 msec, of the response, during which 
charge is transferred inward across the membrane, 
and that this is followed by a gradual dissipation of the 
displaced charge at a rate determined by the electrical 
characteristics of the inactive fiber membrane. It 
agrees with the results of the interaction between the 
endplate potential and action potential from which it 
appears that the charge displacement built up by 
junctional activity can be removed by the high con- 
ductance of the spike at a time shortly following the 
summit of the endplate potential. 

From a knowledge of the membrane capacity for a 
imit length of fil)er, the displacement of charge may 



204 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



be calculated. For an endplate potential of 20 mv 
peak amplitude the maximum displacement of charge 
is found to be about io~' coulombs. Given informa- 
tion on the complete electrical characteristics of the 
fiber, i.e. of the separate values of membrane capacity 
and membrane and core conductances, it is possible 
to analyze more completely the potential wave in 
the fiber. An approximate treatment, in which the 
observed response of the fiber is compared with a 
theoretically derived potential wave for charge placed 
instantaneously at a point along the fiber, confirms 
the above interpretation of the generation of the end- 
plate response by a brief transfer of charge in a small 
area of membrane. 

In a normal uncurarized muscle, the rate of rise of 
the endplate potential is about three times as fast as 
in the above case, due evidently to a proportionately 
more intense transfer of charge. On the endplate 
potential reaching a level of depolarization of about 
40 mv, an action potential is initiated, indicated by a 
sudden increase in the rate of change of potential. 
The threshold for the initiation of an action potential 
has Ijeen examined by the direct application of a 
current pulse, both at the junction and away from it, 
and has been found to occur at all points at this same 
level. The spike which follows the initial depolariza- 
tion produced by the endplate potential is however 
characteristically different at the junction where it 
is evoked from elsewhere in the course of its propaga- 
tion (40, 69; cf. fig. 4}. After rising from the level of 
threshold to zero membrane potential at a rate which 



is not noticeably different in the two cases, the 
junctional spike produces a smaller reversal of mem- 
brane potential than the normal muscle spike away 
from the junction. Thus, at the summit of the junc- 
tional spike the membrane potential is reversed to the 
extent of about 20 mv (total spike height of 1 1 o mv), 
compared to a reversal of about 35 mv for the normal 
spike (total height, 125 mv). The summit of the 
junctional spike occurs earlier and is sharper than 
that of the normal spike. After reaching the summit 
the potential falls to the level of zero membrane 
potential where it remains nearly steady for about 
1.5 msec, before declining further. In contrast, the 
normal spike declines rather slowly for about 2 
msec, after its summit, but then falls more rapidly 
past zero membrane potential. 

It can be shown that these features, which distin- 
guish the junctional spike, do not depend on some 
special characteristic of the action potential process in 
the region where it occurs. When an action potential 
is propagated into the junctional region without the 
nerve having been active, the response is the normal 
muscle action potential similar to that which is elicited 
elsewhere along the fiber. Moreover, these features 
cannot be attributed to the response having originated 
in the region of observation rather than having 
propagated into it, since the propagated action 
potential and the one which is initiated in the region 
of recording by a brief pulse of current show little 
difference beyond the attainment of threshold. It is 
concluded therefore that these features arise from a 




10 msec 



FIG. 3. Endplate potentials recorded intracellularly from a single curarized muscle fiber of a 
frog. The series of five records were taken at intervals of i mm along the fiber. The top record shows 
the response at the junction as inferred from the fact that the response was maximum at this posi- 
tion. [From Fatt & Katz (38).] 

FIG. 4. Action potentials recorded in a muscle fiber in response to a nerve impulse. The upper 
record was taken at the junction, the location of which had earlier been determined by the response 
in the presence of curare. The lower record was taken 2.5 mm away in the same fiber. A trace of 
the endplate potential can still be seen in the lower record, appearing as a gradual rise of potential 
which precedes the foot of the spike. [From Fatt & Katz (38).] 



SKELETAL NEUROMUSCULAR TRANSMISSION 



20 n 



modification of the action potential response by 
junctional activity. The effect of this activity is con- 
sistently to cause a deviation toward a level near 
zero membrane potential. This accounts for the reduc- 
tion in peak amplitude of the spike and the delay on 
the falling phase. As a first approximation it may be 
assumed that the fundamental changes effected in 
the membrane by the two types of activity which are 
superimposed at the junction (i.e. spike and junctional 
activity) do not interact. The effect of transmitter 
action on the spike, as well as the initial development 
of the endplate potential, can then be satisfactorily 
accounted for by an increase in membrane conduct- 
ance in series with an emf set near the level of zero 
membrane potential. However during the action 
potential the situation is complicated by the presence 
in the membrane of two important components of 
conductance, one due to the passage of sodium ions 
and the other to potassium ions, which follow different 
time courses and are dependent on the level of mem- 
brane potential. In order to determine the effect of 
transmitter action more accurately, the spike has been 
set up independentlv of the nerve response by direct 
stimulation of the muscle fiber (24). In this way, using 
a suitably timed nerve impulse, transinitter action was 
made to begin at any chosen stage of the action po- 
tential process, and the resultant deviation of the 
potential observed. It was thus shown that the 
equilibrium potential for junctional activity lies 
between 10 and 20 mv, with the interior of the fiber 
negative. 

The generation of the endplate potential has also 
been studied in the absence of an action potential by 
applying a steady current to the muscle fiber and 
thereby altering the membrane potential at which 
the transmitter operates. .Significant results have been 
obtained only with currents directed inwardly across 
the membrane and causing a hyperpolarization, 
since with currents in the opposite directions complica- 
tions arise owing to the initiation of muscle action 
potentials. The endplate potential was found to vary 
in such a manner as to maintain its rate of rise nearly 
directly proportional to the level of membrane po- 
tential at which it occurred. An equally good fit of 
the data could be obtained with a straight line for 
which zero respon.se would occur at a membrane 
potential of 15 mv, internally negative. There is thus 
complete agreement, as far as the equilibrium value 
is concerned, between the results obtained from the 
effect of junctional activity on the membrane at rest 
and on the membrane undergoing an action po- 
tential. 



In the case of the endplate potential arising in the 
otherwise resting membrane, an analysis has been 
made to determine what size the added conductance 
would have to be to produce the observed rising phase 
of the response. The muscle filler has been treated as 
a cable with known distributive characteristics, and 
the conductance has been considered as applied 
suddenly at a point along this cable. From the change 
of potential occurring in the uncurarized muscle up 
to the level at which the spike is initiated, the con- 
ductance is calculated to correspond to a resistance 
of about 20,000 ohms. This may be considered in 
relation to the resting resistance of about 500,000 
ohms, which is shunted as a result of junctional 
activity, and which is in effect the resistance of the 
membrane over a length of about 4 mm of fiber (twice 
the space constant of the fiijer). An analysis has also 
been made of the effect of junctional activity to reduce 
the reversal of membrane potential at the .summit of 
the spike, together with any additional displacement 
produced by an applied current. The added conduct- 
ance calculated from this information is roughly in 
agreement with the value obtained from the rising 
phase of the endplate potential. 

There appears thus to be a convergence of evidence 
to show that the effect of junctional activity on the 
muscle fiber membrane can be represented as the 
addition of a conductance in scries with a fixed emf 
This may further be interpreted as the creation of a 
new path for the diffusion of ions across the mem- 
brane. The equilibrium value (15 mv, internally 
negative) toward which the membrane potential is 
displaced is the same as the emf that would be 
expected to occur for the unrestricted diffusion of ions 
between two solutions, having the ionic composition 
of the intra- and extracellular media. It is therefore 
concluded that in the new diffusion path created by 
transmitter action, no selectivity is exerted in the 
passage of different ion species other than that already 
existing in the aqueous media on the two sides of the 
membrane. 

The investigations on neuromuscidar transmission 
considered so far in this section have concerned the 
amphibian muscle fibers that under normal conditions 
respond to a nerve impulse with a twitch. The con- 
clusions reached, as to the fundamental alteration in 
the postjunctional membrane produced by the action 
of the transmitter, seem likely to be valid generally for 
junctions on vertebrate skeletal muscle fibers. How- 
ever, marked variations in the overall electrical 
response have been found to occur in different prepa- 
rations, and these are adduced to stem mainly from 



206 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



differences in the electrical characteristics of the 
muscle fiber membrane in parallel with the junc- 
tionally responding region. 

In the mammalian muscle fiber under normal 
conditions, the endplate potential does not form a 
conspicuous step on the rising phase of the spike as it 
does in the amphibian (4). The explanation for this 
lies in the fact that the threshold depolarization for 
initiating an action potential is here much lower 
(about 10 mv compared with 40 mv in the frog), and 
at this level the transition between endplate potential 
and spike does not involve an appreciable change in 
rate of rise of potential. In the curarized preparation 
the response differs from that seen in the frog in hav- 
ing a shorter decaying phase and in becoming at- 
tenuated more rapidly with increasing distance from 
the junction. These differences are attributable 
entirely to a higher conductance of the muscle fiber 
membrane with a consequent reduction in the electric 
time and space constants. 

The tonic muscle fibers of the frog, supplied by the 
small diameter motor nerve fibers, display differences 
in their electrical response from the twitch fibers of 
the same animal which are again mainly attributable 
to the electrical properties of the fiber membrane, 
though the disposition of the nerve endings also plays 
an important part (10, 11, 54). The tonic muscle 
fiber is unable to develop an action potential, due 
apparently to the absence of the mechanism by which 
the sodium permeability of the membrane is increased 
by depolarization. The entire time course of the 
junctional response can therefore be observed under 
all conditions without the complication of a super- 
imposed spike. In addition the amplitude of the junc- 
tional response can be varied by stimulation of differ- 
ent nerve fibers. Owing to a wide and relatively 
uniform distribution of their endings along the muscle 
fiber, the potential wave does not show a marked 
attenuation with distance, and at no position does it 
have the initially rapid and later relatively slow 
decline of the endplate potential recorded at the 
junction of a twitch fiber. Another distinctive feature 
of the response in these fibers is that the membrane 
potential goes through a phase of hyperpolarization 
after recovering from the depolarization. A similar 
phase of hyperpolarization is found to follow a wave 
of depolarization elicited by a current pulse applied 
directly to the muscle fiber, from which it is inferred 
that this feature is not due to .some peculiar charac- 
teristic of the transmission process but depends rather 
on the electrical behavior of the membrane. 



.\CT1VITY OF THE NERVE TERMIN.^LS 

In the preceding .section, the local electrical changes 
brought about in the normal and in the curarized 
muscle fiber by the arrival of an impulse in the pre- 
junctional nerve terminals have been described. In 
this section the behavior of the terminals will be con- 
sidered under various conditions, in .so far as this 
throws light on their specialized properties. Almost 
all the information to be presented is derived from 
recording potential changes in the muscle fiber. 
According to the chemical theory of transmission, 
activity in the nerve terminals causes a release of 
acetylcholine which then reacts with the muscle to 
produce an alteration in it. Hence, when recording 
from the muscle, an indication of activity at the 
terminals is obtained, provided that allowance is 
made for possible effects in the later stages of the 
transmission process. An example of such an effect is 
the reduction of the responsiveness of the muscle 
fiber by curare through its competition with acetyl- 
choline. 

When the membrane potential is recorded in the 
junctional region of a muscle fiber, a sequence of 
small transient changes of potential (as shown in fig. 
5) is observed even in the absence of a nerve impulse 
(3, 41, 62). Although their peak amplitude is only of 
the order of 0.5 mv, these potential changes have 



2raV 




50 msec 



FIG. 5. Spontaneously occurring miniature endplate poten- 
tials recorded at the junctional region of a muscle liber of a 
frog. The location of the recording position was confirmed by 
the form of the response elicited by a nerve impulse. [From 
Fatt & Katz (39).] 



SKELETAL NEUROMUSCULAR TRANSMISSION 



207 



many of the characteristics of a response to a nerve 
impulse. Their time course is similar to the endplate 
potential in a curarized muscle. They appear largest 
at the same place along the muscle fiber and become 
attenuated by changes in the position of the recording 
electrode in the same way. Furthermore, they are 
diminished in amplitude by curare and increased 
and prolonged by anticholinesterases. All the.se fea- 
tures may be accounted for by the properties of the 
postjunctional element and its reaction with acetyl- 
choline. That the nerve terminals are responsible for 
the release of acetylcholine producing these dis- 
charges — called miniature endplate potentials — is 
shown by the fact that they are abolished on nerve 
degeneration and their frequency of occurrence is 
modified by various treatments applied to the nerve. 
In addition there is strong evidence that the end- 
plate potential evoked by a nerve impulse is itself 
resolvable into units of the size of miniature poten- 
tials. 

The miniature discharges occur in a random time 
sequence, the probability of occurrence in any given 
inter\'al of time remaining constant irrespective of 
previous discharges. The distribution of intervals be- 
tween successive discharges is accordingly found to 
follow a simple exponential function, decaying with 
increasing interval, and can be descril)ed by a single 
parameter, the mean frequency of discharge. E.xcep- 
tions to this are occasional bursts which consist of a 
number of miniature endplate potentials occurring 
within a short period of time. They are the only indi- 
cation of a possible coupling between discharges, and 
can be readily recognized and excluded from a sta- 
tistical analysis. In the frog under normal conditions 
the mean frequency of spontaneous discharges varies 
greatly at different junctions, extending at least over 
the range o. i per sec. to 100 per .sec. In mammalian 
muscle the frequency is more nearly constant around 
I per sec. 

The distribution of amplitudes of the miniature 
endplate potentials at a junction can be fitted ap- 
proximately by a Gaussian curve with a standard 
deviation equal to about 30 per cent of the mean. 
With this relatively small variation, the amplitudes 
effectively do not grade down to zero, and hence 
under suitable recording conditions there is no un- 
certainty in counting the discharges. By a variation in 
recording technique, placing the microelectrode in 
contact with the muscle fiber membrane without 
penetrating it, it is possible to restrict the recording 
of miniature discharges to those arising in a small 
fraction of the junctional region contacted by the 



nerve terminals. In this way, one tenth or so of the 
miniature discharges occurring within the fiber are 
recorded selectively while the remainder appear 
greatly attenuated and are in eflPect rejected (27). 
Even under these conditions the amplitude of the 
miniature potentials appears to be continuously 
distributed, there being no clear indication of a 
number of discrete .sizes which are repeated. 

A notable feature of the miniature discharge is 
that the release of acetylcholine which produces it 
does not appear to change under various treatments 
which have an important influence on the genera- 
tion of an electrical response (28). Even in the situa- 
tion where the nerve and muscle membranes have 
been completely depolarized i)y a high concentra- 
tion of potassiimi ions, it can be shown by repolarizing 
the muscle fiber with an applied current that the 
intermittent release of sinall quantities of acetyl- 
choline, capable of producing miniature potentials, 
still occurs (26). It is therefore concluded that the 
release of acetylcholine forming these discharges does 
not depend upon the occurrence of electrical activity 
of the action potential type in any structural unit 
within the nerve terminal. 

Unlike the amplitude (considered as a quantity of 
acetylcholine released from the terminal), the fre- 
quency of the spontaneous discharges is highly sensi- 
tive to changes in the condition of the preparation. 
Changes in the osmotic pressure of the surrounding 
fluid, for example, have a strong effect, the frequency 
increasing reversibly as this is raised (41, 44, 62). A 
finding which is important in indicating a possible re- 
lation ijetween electrical events in the nerve and these 
spontaneous discharges is that their frequency can 
be altered by the application of a current to the 
nerve which, by spreading into the terminal portion, 
will alter the membrane polarization there (23, 64). 
The frequency is found to vary approximately ex- 
ponentially with changes in the polarizing current in 
the nerve, being increased by depolarization of the 
terminals. The frequency of discharge is also increased 
when the concentration of potassium ions in the 
bathing fluid is raised above the normal level, this 
probably operating in the same way as current by- 
causing a reduction of membrane potential. 

The rate of rise of the endplate potential, up to the 
level at which an action potential is initiated, is 
about one hundred times greater than the mean rate 
of ri.se of the miniature endplate potential. A decrease 
in the calcium ion concentration of the solution bath- 
ing the preparation causes a reduction in the endplate 
potential, while the amplitude of the spontaneous 



208 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



discharge is left unchanged; the same effect is pro- 
duced by the addition of magnesium ions (4, 19, 
20, 21, 41, 63). Calcium appears to exert a specific 
facilitatory action on the release of acetylcholine by a 
nerv'e impulse, and the action of magnesium may 
then be accounted for by a competition with calcium 
for the reactive site. This antagonistic relation be- 
tween calcium and magnesium at the terminals is in 
contrast to their common action in raising the 
threshold depolarization for the initiation of an action 
potential in a nerve or muscle fiber. By the with- 
drawal of calcium or addition of magnesium or both, 
the endplate potential can be reduced to a small 
fraction of its normal size and can be made to ap- 
proach in amplitude the spontaneous miniature po- 
tential. When this is done, the amplitude of the 
response to successive nerve impulses is seen to fluc- 
tuate widely, in contrast to its constancy under 
normal conditions or when the response is reduced to 
any degree by treatment with curare. With the 
junction sufficiently deprived of calciimi, the response 
occurs intermittently. When the proportion of fail- 
ures is large, the responses to a series of ner\e im- 
pulses have a distribution of amplitudes similar to 
that of the spontaneous discharge. At a somewhat 
lower level of depression, the distribution shows 
several peaks, corresponding to small integral multi- 
ples of the mean of the spontaneous potentials. It is 
evident that the endplate potential under these condi- 
tions is composed of a variable whole number of 
miniature endplate potentials, the fluctuation being 
due to variation in number and in size of units. The 
result of an analysis of this fluctuation for the proba- 
bility of occurrence of different numbers of units can 
be accurately fitted by a Poisson distribution. This 
implies that there is no interaction between units, 
the probability of occurrence of each being unaffected 
by the number of units composing the response. 

With the distribution in this form, relatively large 
fluctuations in response would occur only when the 
number of contributing units is small. As the number 
increases, the amplitude of fluctuation relative to the 
mean amplitude of response will vary in inverse 
proportion to the square root of the number of units, 
while the additional dispersion due to variation in 
amplitude of individual units will become pro- 
gressively less significant. Fluctuations occur in the 
curarized endplate potential, evoked under condi- 
tions in which the release of acetylcholine from the 
nerve terminals is normal, and these can be attributed 
to a variation in the number of units around such a 
magnitude as would be predicted roughly from the 



size of the normal response (68). The probability 
that the normal endplate potential is composed of 
these units is greatly strengthened by the observa- 
tion that an increase in the calcium ion concentra- 
tion beyond that normally present in the bathing 
fluid produces a further increase in the size of the 
response, which is entirely attributable to an increase 
in the release of acetylcholine from the nerve ter- 
minals, and which is presumably due to an increase 
in the probability of release of individual units of 
acetylcholine (13, 29, 43, 52). The curarized end- 
plate potential can in this way be increased two or 
three times in size. 

In contrast with the effect on the response to a 
nerve impulse, changes in the calcium concentration 
in cither direction from normal are usually found 
to have no effect on the frequency of spontaneous 
miniature potentials. Calcium withdrawal (or mag- 
nesium addition) does, however, reduce the fre- 
quency when this has first been raised by the presence 
of a high concentration of potassium ions or by a 
current applied to depolarize the nerve terminals. It 
thus appears that the depletion of calcium ions has a 
similar action in presenting an increase in the proba- 
bility of a unit of acetylcholine being released during 
a given time interval by a maintained depolariza- 
tion, as in reducing the probability of its release by a 
nerve impulse. 

Another procedure which modifies the number of 
units responding to a nerve impulse is the previous 
activation of the nerve. In the curarized amphibian 
muscle, the second of two closely spaced nerve im- 
pulses elicits a larger endplate potential than does the 
first (30, 43, 72). With continued repetitive stimula- 
tion of the nerve, the individual responses increase 
progressively until a steady condition is attained. By 
this means the size of the response may be increased 
to two or three times that elicited by an isolated im- 
pulse. (This increase is in addition to the summation of 
electrical changes in the postjunctional structure, the 
later responses adding to the potential change remain- 
ing from previous responses.) In the case where two im- 
pulses are set up in the nerve the potentiation of the 
response to the second is found to be greatest at the 
shortest interval of time at which the ners-e will con- 
duct. The effect falls gradually as the interval be- 
tween the nerve impulses is increased, the response 
having returned to its unpotentiated size at an inter- 
val of about 100 msec. That this potentiation is a 
prejunctional phenomenon and moreover that it in- 
volves a change in the number of units of acetylcho- 
line released is revealed by studying the effect under 



SKELETAL NEUROMUSCULAR TRANSMISSION 



209 



conditions in which the number of units responding 
to a nerve impulse is small. For this purpose the cal- 
cium concentration is reduced (or magnesium added) 
until the response to a single nerve impulse has a 
mean amplitude of one or a few units. With two 
nerve impulses at a short interval apart the response 
to the second is found to be statistically larger, as in 
the curarized preparation. Examination of the distri- 
bution of amplitudes for the first and second re- 
sponses in a number of trials reveals that the increase 
in the second is accompanied by a reduction in its 
fluctuation, indicating that the change is entirely 
the result of an increase in the number of units re- 
sponding (22). It is further found that the number of 
units responding to the first nerve impulse in a par- 
ticular trial has no effect on the number responding 
to the second in that trial. This leads to the conclusion 
that the potentiation of the second response depends 
solely on the previous presence of an impulse in the 
nerve and not on the number of units of acetylcholine 
released by the impulse. 

Whereas in the amphibian the second of two 
ner\e impulses elicits an endplate potential which is 
larger than the first, in the curarized mammalian 
preparation the response to the second is smaller up 
to an interval of a few seconds (30, 65, 66). Evidence 
of potentiation by previous activity of the nerve is 
procured where the conditioning treatment is a large 
number of ner\e impulses. When between a few 
hundred and a few thousand impulses are set up in 
the nerve within 5 to 20 sec, the later impulses in the 
train elicit a considerably reduced response owing to 
the depressant efifect of preceding volleys. The time 
course of subsequent changes in the effectiveness of 
transmission is revealed by testing with a single im- 
pulse at a variable time after the termination of the 
conditioning train of impulses. It is thus found that 
the effectiveness of transmission gradually increases 
from the depressed state to beyond that occurring in 
the absence of previous activity (5, 48, 65). The mag- 
nitude and time course of this potentiation depends 
on the number of conditioning nerve impulses; it is 
larger, arises later and is more prolonged, the greater 
the number of impulses. Following a few thousand 
impulses, the maximum is not reached until about 0.5 
min. after conditioning, when the response as meas- 
ured by the size of the endplate potential may be 50 
per cent greater than the normal and the total dura- 
tion of the potentiated state may be i o min. 

When the curare-free mammalian preparation is 
subjected to calciuin depletion, a behavior is observed 
which is similar to that in the frog. The second of two 



closely spaced nerve impulses now elicits a greater 
response than the first (67). The effect of condition- 
ing with a train of impulses is to cause a summation 
of the potentiation left behind by individual nerve 
impulses. It is apparent that the potentiation in the 
wake of a nerve impulse has a very prolonged phase 
of low level effectiveness, which, while hardly notice- 
able after a single impulse, is able to sum over a large 
number of impulses to produce an appreciable po- 
tentiation of very great duration. When the calcium 
concentration is normal, the earlier part of this po- 
tentiation is outweighed by the depression which 
follows each nerve impulse but does not sum over as 
long a period of time. The fact that the depression 
does not occur in the calcium depleted preparation 
when the number of units of acetylcholine released by 
each impulse is small makes it appear highly probable 
that this effect, unlike the potentiation, depends on 
the amount of acetylcholine released by previous 
impulses. 

In the mammalian muscle under normal condi- 
tions, the frequency of spontaneous discharges is 
found to be increased immediately following the 
response to a conditioning nerve impulse at which 
time the response to a second impulse is diminished. 
After conditioning with a large number of impulses, 
the frequency is increased many times and returns 
only very slowly to normal (6, 62). The final part of 
its return parallels the time course of the subsidence 
of the potentiation of transmission, as observed in the 
curarized muscle. The effect of previous activity of the 
nerve is apparently to increase the potentiality of the 
terminals for releasing units of acetylcholine, both 
spontaneously and in response to a nerve impulse. 



PROPERTIES OF THE JUNCTIONAL RECEPTOR 

The most direct method for investigating the 
receptive properties of the muscle fiber is to add acetyl- 
choline to the surrounding fluid without involving 
the nerve terminals. Two techniques have been used: 
the acetylcholine has been applied either uniformly 
to the whole muscle fiber, or in a highly localized 
manner to the region contacted by the nerve endings. 
The effect is a depolarization of the muscle fiber in 
the junctional region (12, 17, 36, 51). After pre- 
liminary treatment with an anticholinesterase, which 
prevents the enzymatic destruction of acetylcholine, 
the technique of uniform application allows quantita- 
tive information to be obtained on the reactivity of 
the receptor with varying concentrations of acetyl- 



210 



HANDBOOK OF PHYSIOLOGY 



.NEUROPHYSIOLOGY I 



choline. When the acetylcholine concentration is as 
high as I jumole per liter, muscle fibers are depolarized 
sufficiently for spikes to be initiated. For low concen- 
trations, not exceeding that required to elicit spikes, 
the depolarization is nearly proportional to the acetyl- 
choline concentration. With high concentrations the 
depolarization elicited by acetylcholine can be meas- 
ured in the wake of an initial burst of spikes, when 
the muscle fiber in the region of the junction is re- 
fractory to the initiation of further spikes. At the 
lower concentrations the depolarization is maintained 
for many minutes while the acetylcholine remains in 
the surrounding fluid; at the higher concentrations a 
perceptible decline is observed within a few minutes, 
the rate of decline being greater the higher the con- 
centration of acetylcholine. This effect is apparently 
the result of a gradual desensitization of the receptor 
by its forming a different and less readily reversible 
combination with acetylcholine than that which 
results in depolarization. 

More accurate information on the spatial distribu- 
tion of the receptor and the time course of its reaction 
may be obtained by applying brief pulses of acetyl- 
choline with a micropipette (25, 70). It is found that 
the high sensitivity to acetylcholine does not extend 
beyond very limited regions in the neighborhood of 
the nerve terminal branches, for in the frog, where the 
terminals are spread over about a 200 /n length of fiber, 
it is necessary to position the micropipette to within 
10 or 20 M in order to obtain a high sensitivity. It is 
further observed that acetylcholine exerts its power- 
ful action only when applied externally; it has no 
specific effect when released within the muscle fiber, 
even though the pipette is situated immediately be- 
neath a region of the fiber surface that is found to be 
sensitive to external application. With the micro- 
pipette critically placed over the junction so as to 
obtain maximum sensitivity, the depolarization 
evoked by a brief pulse of acetylcholine rises to a peak 
in about 15 msec. This order of lime would no doubt 
be required for the diffusion of acetylcholine from its 
point of release to the receptor some microns away. 

Among agents that affect neuromuscular trans- 
mission, the one that has received most attention is 
curare. This term applies to a group of related sub- 
stances which act by competing with acetylcholine 
for the receptor. Combination of curare with the re- 
ceptor does not itself aflfect the electrical properties of 
the membrane, but it prevents acetylcholine combin- 
ing and thereby exerting a depolarizing action, .\mong 
the common inorganic ions, sodium appears to have 
the most marked effect on the combination of acetvl- 



choline with the receptor (36, 42). After the complete 
withdrawal of sodium ions from the bathing solu- 
tion, the application of acetylcholine elicits a small 
depolarization, which is augmented considerably by 
the presence of only a small concentration of sodium. 
This effect is not produced by the addition of calcium 
or potassium ions. It is inferred to be due to a change 
in the reaction between the receptor and acetylcholine, 
rather than in a later stage of the process leading to 
depolarization, from the fact that sodium ions also in- 
crease the ability of acetylcholine to compete with 
curare for the receptor. A facilitation of the reaction 
between the receptor and acetylcholine in muscles of 
the frog is also produced by the addition to the 
bathing medium of very small concentrations of epi- 
nephrine and norepinephrine, the substances released 
by impulses at the terminals of sympathetic post- 
ganglionic nerve fibers (49}. 

The anticholinesterases are a group of substances 
that affect transmission by competitively inhibiting 
the enzyme cholinesterase, which is concentrated in 
the junctional region of the muscle fiber and normally 
hydrolyzes acetylcholine soon after its liberation from 
the nerve terminals. Unlike the reaction between the 
receptor and acetylcholine or curare, which must be 
very rapid in reaching an equilibrium, that between 
the enzyme and a reversible anticholinesterase takes 
many minutes. With the anticholinesterase exerting 
its maximum effect and presumably completely in- 
hibiting the enzyme, the time course of transmitter 
action is in two stages (31, 40). The 2 msec, phase of 
high intensity transmitter action is virtually un- 
changed and accounts for the early rapid rise of the 
endplate potential. This is succeeded by a prolonged 
phase of low level transmitter action which heightens 
and prolongs the endplate potential. 

Other organic compounds besides acetylcholine 
exert a depolarizing action at the junction. Some of 
the substances that have been examined combine in 
various degrees the properties of acetylcholine, curare 
and anticholinesterases (32, 74). In the case where 
the first two actions are combined, the agent in a 
concentration which produces a small depolariza- 
tion prevents acetylcholine from adding to this to the 
extent obtaining when the former is absent. Different 
substances are found to follow various time courses 
in their action, and where the same one exerts multi- 
ple types of action, each may develop along a different 
time course. Furthermore the relative effectiveness for 
each type of action may vary between different 
preparations. 

Transmission would be expected to be influenced at 



SKELETAL NEUROMUSCULAR TRANSMISSION 



various stages by changes of temperature. The most 
conspicuous result of lowering it is a prolongation of 
the phase of transmitter action. This appears to be 
due largely to a reduction in the activit) of cholin- 
esterase since at low temperatures treatment with an 
anticholinesterase produces little additional change 
(4, 31). It is found, however, that, while the time 
course of the curarized endplate potential is length- 
ened, the peak amplitude is not significantly in- 
creased as it should be if the early phase of transmitter 
action were unaltered. In the mammalian muscle this 
appears to be the result of curare competing more 
effectively with acetylcholine at the reduced tempera- 
ture and thus ofTsetting the effect of the reduction in 
cholinesterase activity on the peak potential change. 
An experiment, highly relevant to the conclusion 
that the alteration of the properties of the muscle 
fiber produced by a nerve impulse is consistent with 
the operation of a chemical mediator, is the demon- 
stration that the depolarization elicited by acetyl- 
choline has its origin in the same conductance change 
that has been shown to occur during transmission 
(26). For this purpose the muscle has first been 
nearly completely depolarized by immersing it in a 
solution with a high concentration of potassium ions. 
In this condition the application of acetylcholine pro- 
duces no discernible change in inembrane potential. 
When the membrane is now polarized in either direc- 
tion by the passage of current across it, acetylcholine 
produces a potential change that partly compensates 
for the displacement from the unpolarized state, and 
this is attributable to an increase in membrane con- 
ductance similar to that observed for the preparation 
initially in its normal environment. 



CONCLUSION : MECHANISM OF TR.ANSMISSION 

From the rate at which acetylcholine appears in the 
effluent from a perfused muscle during repetitive 
stimulation of the motor nerve fibers, it has been es- 
timated that the quantity released from the nerve 
endings at a single junction in response to a single 
nerve impulse is about io~'- moles (i, 35). Although 
the value obtained in this way is liable to be too small 
because of losses in the collection procedure and be- 
cause of a depression in the release mechanism by 
previous activity, it is not likely to be in error in its 
order of magnitude. It may be compared with the 
minimum quantity of about 5 X lo""' moles of acetyl- 
choline which is required to evoke a muscle action 
potential when applied to the junctional region by a 



micropipette (25, 70). The factor of 200 between these 
two quantities can be satisfactorily accounted for by 
the geometry of the junction. The nerve endings from 
which the acetylcholine is released are probably every- 
where in very close proximity to the receptive region 
of the postjunctional surface with a consequent high 
efficiency for its reaching the receptor. On the other 
hand, when acetylcholine is applied by a micro- 
pipette, it would have to diffuse over a greater distance 
and be considerably dispersed before reacting with 
the receptor, and a larger quantity' would therefore 
be required to produce a comparable effect. Even if 
the micropipette were placed directly on a sensitive 
region, the application of a moderate amount of 
acetylcholine would no doubt lead to a rapid satura- 
tion and inactivation of the receptor there because of 
its high local concentration, and the initiation of an 
action potential would require the action of acetyl- 
choline over a greater part of the receptive area. 

From the concentration of acetylcholine required to 
produce an action potential when applied uniformly 
to the preparation and from the quantity that is re- 
leased by a nerve impulse, it is possible to calculate 
the volume in which the acetylcholine released from 
the nerve terminals would be present when reacting 
with the receptor (37). The result shows that the 
acetylcholine must exert its maximum effect before 
diffusing more than i //, a distance which is consistent 
with morphological findings on the minute separation 
of the pre- and postjunctional surfaces. Furthermore, 
assuming that diffusion occurs away from the im- 
mediate neighborhood of the junction, the time dur- 
ing which the acetylcholine will remain in an effective 
concentration is shown to be less than i msec. The 
brief duration of transmitter action may reflect the 
operation of this diffusion, though the possibility re- 
mains that the reaction between the receptor and 
acetylcholine does not reach an equilibrium in such a 
short period of time and the kinetics of this reaction 
may then influence the time course of transmitter 
action. At least it is clear that the enzymatic destruc- 
tion of acetylcholine is not involved in the early, high 
intensity phase of transmitter action, as it is not 
affected by the presence of an anticholinesterase. The 
failure of the destruction of acetylcholine adds a 
later low level phase of transmitter action which 
probably occurs after the acetylcholine has diffused 
away from the immediate neighborhood of the 
terminals where it is released and is dispersed over 
the entire junctional region. 

The high degree of chemical specificity of the 
receptor and the competition for it of different sub- 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



Stances with different final effects is suggestive of the 
behavior of an enzyme. It is highly relevant to this 
that substances which are able to displace acetyl- 
choline from the enzyme cholinesterase are also able 
to displace it from the receptor. The receptor appears 
almost certainly to be a protein constituent of the 
muscle fiber membrane with its reactive sites exposed 
on the outer surface. As a result of the combination 
of these sites with acetylcholine, the physical proper- 
ties of the membrane alter and a new path appears 
for the diffusion of ions of various species through it. 
In electrical terms transmitter action may be ap- 
proximately described as the placing of an addi- 
tional conductance across the membrane which short- 
circuits any previously existing potential difference. 
In that the experimental findings are in agreement 
with this interpretation, they exclude the possibility 
of electrical transmission by which the junctional 
response is considered to be produced by an externally 



generated current impressed upon the muscle fiber. 
At the same time they eliminate the possibility that 
the response may be of the nature of a local response, 
a specific increase in membrane permeability to 
sodium ions boosting an initially small potential 
change, such as may occur when the membrane is 
depolarized to near the threshold for setting up an 
action potential. It appears that the junctional 
respon.se cannot be brought about by any means of 
electrical stimulation of the postjunctional structure 
but only by a specific chemical reaction of the re- 
ceptor. The presence at the junction of a region capa- 
ble of responding in this way does not appear to 
affect the action potential developed there, except 
by an addition of the independent actions of the two 
types of activity. The probable significance of this is 
that the area occupied by the receptor is small and 
does not detract appreciably from the area engaged 
in producing the action potential. 



REFERENCES 



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Bernard, C. Lemons sur les effels de substances toxiques et 

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Boyd, I. A. and A. R. Martin. J. Physiol. 132: 74, 1956. 

Boyd, T. E. Am. J. Physiol. 100: 569, 1932. 

Brooks, V. B. J. Physiol. 1 34 : 264, 1 956. 

Brown, G. L. J. Physiol. 89: 220, 1937. 

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Brown, G. L., H. H. Dale and W. Feldberg. J. Phynoi 

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Burke, W. and B. L. Ginsborg. J. Physiol. 132: 586, 1956. 
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Cowan, S. L. J. Physwl. 88: 3P, 1936. 

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30. EccLES, J. C, B. Katz and .S. W. Kuffler. J. .Neuro- 
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SKELETAL NEUROMUSCULAR TRANSMISSION 



213 



55. KUFFLER, S. W. AND E. M. Vaughan VVilliams. J. 
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CHAPTER VII 



Autonomic neuroefFector transmission 



U. S. V O N E U L E R I Department of Physiology, Faculty of Medicine, Stockholm, Sweden 



CHAPTER CONTENTS 

Development of the Concept 
Anatomical Considerations 
Humoral Versus Electrical Transmission 
T'^e Adrenergic Nerve Transmitter 
dentification 
Occurrence, Biosynthesis and Storage of Adrenergic Nerve 

Transmitter 
Release 

Influence of stimulation frequency 
Effects on remote organs 
Stimulation of isolated nerves 
Exhaustibility 
Removal of Transmitter 
Possible Adrenergic Nerve Transmitters Other Than Norepi- 
nephrine 
The Cholinergic Nerve Transmitter 
Identification 

Occurrence, Biosynthesis and Storage 
Release in Organs 

Release from isolated nerves 
Removal of Transmitter 
Mechanism of Action of Neurotransmitters 
Neurotransmitters in Blood and Urine 



DEVELOPMENT OF THE CONCEPT 

THE IDEA OF CHEMICAL TRANSMISSION of nerve im- 
pulses was apparently first expressed by Elliott (41) 
who in 1904 suggested the possibility that when the 
sympathetic nerve impulse reached the target cell it 
caused an action by liberating epinephrine "on each 
occasion when the impulse arrives at the periphery." 
This hypothesis was based on the similarities in action 
of epinephrine and sympathetic nerve activity, 
irrespective of whether the action was activation or 
inhibition. 

Elliott's idea, although representing an entirely 
new concept, must have struck many as plausible, and 
it was not surprising that thinking should proceed 



along similar lines. Thus Dixon & Hamill (36) ap- 
plied the idea to parasyinpathetic nerves, comparing 
their action with that of muscarine, and after this 
time it became primarily a matter of skillful experi- 
mentation to prove the correctness of the theory and 
to carry the new concept to general acceptance. This 
task proved more difficult than was perhaps antici- 
pated. It was chiefly due to the precision of observa- 
tion and judgment of Dale (25) and the ingenious 
experimentation of Loewi (83) that the postulate of 
chemical transmission became tran.sformed into an 
accepted concept. Acetylcholine gradually moved into 
the center of interest as a possible candidate for 
parasympathetic nerve transmission. In Dale's paper 
concerning the action of injected acetylcholine, he 
stated that it caused "pronounced vagus-like inhibi- 
tion of the heart, and various other effects of stimu- 
lating nerves of the cranial and sacral divisions of the 
autonomic system — secretion of saliva, contraction of 
the oesophagus, stomach and intestine and of the 
urinary bladder." 

The direct experimental proof was provided by 
Loewi (83) who showed that the fluid collected from 
an isolated frog's heart during vagus stiinulation 
inhibited a second heart (fig. i). The effect of the 
"Vagusstoff" was annulled by atropine and in a large 
series of experiments it could be shown that the 
liberated substance behaved in every respect, phar- 
macologically and chemically, like a choline ester. It 
is generally assumed that it is acetylcholine. 

Stimulation of the sympathetic nerves in Loewi's 
experiments caused the release of a factor which 
accelerated the heart and had properties similar to 
those of epinephrine. Chemical transmission of 
sympathetic nerve impulses was independently dem- 
onstrated by Cannon & Uridil (21) who found that 
the stimulation of hepatic nerves increased the rate 
of the denervated heart and rai^d the arterial pres- 



215 



2l6 HANDBOOK OF PHYSIOLOGY -^ NEUROPHYSIOLOGY I 




liliililLMi 



.UiMJviiiUi 



FIG. I. Bain's modification of the original experiment performed by Loevvi in 1921. The diagram 
represents a reservoir of salt solution from which there is a passage to the donor heart (D); pressure 
trom the reservoir assures a continuous flow of the solution through that heart to the recipient heart 
(R). The donor heart still has its proper nerves. Each heart is attached to a writing lever. The record 
is that of the two hearts, donor and recipient. When the vagal fibers of the donor were stimulated (S), 
there was a prompt arrest of that heart (D), and later a slowing and arrest of the recipient heart 
(R), with gradual recovery. Time (T) is recorded in 5-sec. intervals. [From Bain (7).] 




FIG. 2. Rise of arterial pressure and increase of heart rate from 
196 to 220 beats per min. following stimulation of the hepatic 
nerves in the cat. Time, 5 sec. [From Cannon & Uridil (21).] 



sure (fig. 2). It did not dilate the pupil, howexer, 
which would have been expected if the substance 
carried by the blood were epinephrine. 

The principle of chemical transmission was later 
greatly developed chiefly by the work of Cannon & 
Rosenblueth and their associates, and by Dale, 
Feldberg, Minz and their co-workers. A very useful 



distinction was introduced by Dale (27) when the 
tcrins adrenergic and cholinergic nerves were coined 
(fig. 3). While acetylcholine still holds the position 
allotted to it since 191 4 as the cholinergic chemotrans- 
mitter, the concept of epinephrine as adrenergic 
transmitter has had to yield to its nonmethylated 
homolosiue norepinephrine (124). The "curiously 
anomalous" effect on the iris observed by Cannon & 
Uridil in 1921 (21) i:)ecame readilv explained by the 
recognition that norepinephrine and not epinephrine 
was the mediator of adrenergic nerve action. 

For a detailed account of the problem of autonomic 
neuroeffector transmission the reader is referred to the 
monographs of Gaddum (50), Cannon & Ro.scn- 
blueth (20), Muralt (133), Rosenblueth (113) 
Minz (96, 97), Euler (129) and the recent survey of 
neurochemistry (loi). 



.^N.ATOMIC.^L CONSIDER.ATIONS 

As in other tields of ph\siolog"y, valuable hints may 
be gained by studying the microarchitecture of the 



AUTONOMIC NEUROEFFECTOR TRANSMISSION 



217 



region in question, in this case the structural relation- 
ships between the autonomic postganglionic nerve 
endings and the target cells. These cells in principle 
include the heart muscle cells and the secretory cells 
of the glands in addition to those of smooth muscle. 
Much conflicting evidence has been presented with 
regard to the innervation of smooth muscle cells by 
autonomic ner\e fibers. It inay sufhce to mention that 
an histologist as experienced as Slohr (121) found 
that less than one cell in a hundred was inner\ated. 
The numerous reports on intracellular nerve endings 
in smooth muscle cells seem to require reconsideration 
since an ingrowth of axonal endings into a cell ap- 
pears for many reasons unlikely, and even unneces- 
sary, especially in view of the probable distribution of 
the transmitter in the terminal parts of the axons, 
to be discussed later. It must therefore be seriously 
considered whether the alleged findings are not due to 
misinterpretation of the histological pictures. It is 
well known that smooth muscle cells may .serve their 
proper function without innervation, and unless it 
can be shown that each smooth muscle cell receives 
intracellular nerve twigs there is every reason to 
regard the few exceptions known at present as interest- 
ing special cases of unknown functional significance. 
The finding of numerous endings in the ciliary muscle 
of the eye does not alter the general picture. There is 
nothing known so far to indicate any kind of motor 
end plate' on the sinooth muscle cell. Knoblike 
thickenings ending at or near the cell surface have 
been described, however, both by older histologists 
and more recently. Similar structures, sometimes 
assuming the picture of bead-strings, have been re- 
peatedly found at autonomic nerve endings (54, 62, 
72). Garven & Gairns suggest "that the small beads 
on the course of the finest fibrils represent the actual 
release points of the humoral agents within the cyto- 
plasmic continuum provided by cells other than the 
neurones. " 

As will be discussed in the following section the 
results of studies of electrical phenoinena in the 
siTiooth muscles do not suggest direct innervation of 
such cells. 

Cannon & Rosenblueth (20) have regarded the 
few innervated cells as having special functions and 
have named them 'key cells.' Their contention was 
that by chemical transmission concentrated to these, 
the neighboring cells will be affected by the diflfusing 
neurotransmitter. There is little evidence to support 
this hypothesis, however. Moreover, since it is known 
that the autonomic nerve trans mitters are p resept 




FIG. 3. Dale's schpmatic representation of the autonomic 
nervous system. A, adrenergic; C, cholinergic elements. 
[From Dale (28).] 

all along the axons, it is unlikely that they should 
be released only at one point of the axon in a small 
nuinber of special cells. 

The question of the innervation of the smooth mus- 
cle cell cannot be answered with coinplete certainty 
but the best evidence points at a peripheral branching 
system of the postganglionic autonomic nerve fibers 
extending to the inynediate neighborhood of each 
snipoth muscle cell (62). By release of the chemical 
transmitter during nerve stimulation, the cells will 
be reached by the active chemical substance through 
diffusion. The proportion of cells activated in an organ 
and the degree of activation will clearly depend upon 
the amount of transmitter set free, which in its turn 
is a function of the frequency and strength of the 
stimulus applied to the nerve. 



HUMORAL VERSUS ELECTRICAL TRANSMISSION 

The bulk of evidence points to the conclusion that 
denervated smooth muscle is electrically inexcitable 
(100, 114). Even if direct stimulation of denervated 
smooth iTiuscle may lead to contraction, this is weak 
and differs in several respects from that produced by 
the chemical stimuli. It appears likely that the direct 
stimulation effect is unspecific and due to a direct 
gross action on the contractile material. An important 



2l8 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



argument is further that single stimuli are not capable 
of eliciting contractions. Whether the negative results 
of stimulating the denervated adrenal medulla (114) 
can be used as support for the thesis of inexcitability of 
denervated target cells is open to doubt. 

The inexcitability of autonomic effectors has also 
been studied by 'chemical denervation,' by use of 
drugs which block the action of the autonomic nerves 
on the target cells. Such experiments have been made 
on the piloerectors after ergotoxin (20) and on the 
salivary gland cells after chlorpromazine (42). 

It may therefore be concluded that the smooth mus- 
cle cell lacks the ability to respond to direct electrical 
stimulation. Since there is ample evidence to show 
that these cells respond readily to the chemical 
stimuli which are known to Ije released from the 
terminal parts of the autonomic postganglionic nerves, 
there .seems to be no need to postulate electrical 
transmission for functional reasons. 

For a detailed discussion of the dual theory of 
chemical and electrical transmission advocated by 
Monnier & Bacq (100) see Cannon & Rosenblueth 
(20}. While there is no evidence for electrical trans- 
mission from the postganglionic autonomic nerve 
fiber to the effector cell, the situation may be different 
in the case of autonomic synapses (loi). 

Smooth muscle thus differs fundamentally from 
skeletal mu.scle in that the latter is rapidly activated 
by a trigger mechanism requiring direct contact i:)e- 
tween the nerve fiber and the effector and working on 
the all-or-none principle. The sustained activity of 
the smooth muscle appears to operate on the entirely 
different principle of graded responses (115). More 
data are required, however, before the activity of the 
single smooth muscle cell in response to physiological 
stimuli can be ascertained. 



by biological tests and by colorimetric methods (1^9)- 
For the identification of the transmitter the differ- 
entiation from epinephrine became of primary im- 
portance. On most target cells the actions of epineph- 
rine and norepinephrine are qualitatively similar, 
but the relative activity varies from one organ to 
another. Thus the action of epinephrine may be 
over one hundred times that of norepinephrine on the 
rat's uterus and on the fowl's rectal cecum while the 
two amines ha\e about the same activity on the iso- 
lated heart. By comparing the actions of the purified 
extracts containing the neurotransmitter on a series 
of test preparations it is possible to ascertain whether 
the relative actions of the unknown compound go 
parallel with one or the other of the standard sub- 
stances. Though norepinephrine passed unnoticed 
by chemical tests in the so-called pure crystalline 
epinephrine prepared from suprarenals for nearly 
50 years, the amines are now readily separated by 
chromatography (73). 

Generally a single pair of test objects showing 
sufficiently large differences in the activity' ratio be- 
tween epinephrine and norepinephrine suffice for 
differentiation between the two amines. Suitable 
pairs are for instance the cat's arterial pressure and 
the fowl's rectal cecum. On the former preparation 
norepinephrine is from i to 5 times more active as a 
pressor agent than epinephrine, while it has only ' 4 
to J200 of the activity of epinephrine on the fowl's 
rectal cecum (fig. 4). 

The virgin uterus of the cat, and the iris are 5 to 10 
times more sensitive to epinephrine than to norepi- 
nephrine and may be u.sed for differentiating pur- 
poses. The rat's uterus under certain conditions is 
stimulated by norepinephrine and relaxed by epi- 
nephrine (fig. 5). 



THE ADRENERGIC NERVE TRANSMITTER 

Identification 

As outlined in the introductory section, Loewi's 
experiments in 1921 supported the idea that the 
sympathetic (adrenergic) transmitter was epineph- 
rine-like. The suggestions by Barger & Dale (9), Bacq 
(4) and Greer, Pinkston, Baxter & Brannon (58) that 
norepinephrine conformed better with the actions of 
the sympathetic transmitter than did epinephrine re- 
ceived little attention until it was shown by von Euler 
(124) that the adrenergic nerves contained not epi- 
nephrine but norepinephrine. The identification of 
the transmitter with le\'o-norepinephrine was proved 




FIG. 4. Effect of epinepfirine (/-adr), norepinephrine (/- 
nor-adr) and extract of beef splenic nerves (Spl. n.) on the 
arterial pressure of the cat and on the isolated rectal cecum of 

the fowl. [From von Euler (128).] 



AUTONOMIC NEUROEFFECTOR TRANSMISSION 



219 




0.2 ng 0.2 (IF, 0.1 ug 0.2 |ig 

Koradr. Adr. Ijore.dr. Adr. 

FIG. 5. Rat uterus, 3 hours post partum. o.i and 0.2 Mg 
norepinephrine stimulates, 0.2 Mg epinephrine inhibits the 
uterus. [From GreetT & Hokz (56).] 



It has been observed for some time that although 
the actions of epinephrine on the arterial pressure of 
the cat may be reversed by antisympathomimetic 
substances (ergotoxine, yohimbine, benzodioxane, 
dibenamine, phentolamine), the effects of sympa- 
thetic nerve stimulation are at the most weakened or 
annulled but never reversed. The explanation was 
obtained when it was observed that the action of 
norepinephrine on the arterial pressure is not re- 
versed but only diminished by doses which reverse 
the action of epinephrine. This difference has been 
utilized for the classification of the adrenergic neuro- 
transmittor both in vitro (124, 129) and in vivo (20, 48). 

The identification has subsequently been con- 
firmed by other methods, notably by paper and 
column chromatography, allowing separation from 
other catechol amines and characterization by 
specific color or fluorescence reactions. Extracts of 
heart yield fractions on column chromatography 
which show the same R-value as pure norepinephrine 
and the same biological actions (55)- A particularly 
good source of the adrenergic transmitter is the 
splenic nerves, from which norepinephrine can be 
separated by column chromatography and identified 
by location and by analysis of the active fractions 
(fig. 6). Venous blood from the spleen collected dur- 



ing stimulation of the adrenergic nerves contains 
practically pure norepinephrine (98, 108). 

The effects of reflex activation of sympathetic 
nerves as well as the effects of direct nerve stimulation 
show all the characteristics of norepinephrine actions 
(9. 48, 52, 58)- 

The release of an active substance on stimulation 
of the nerves to an organ does not necessarily mean 
that this substance is the corresponding chemotrans- 
mitter. In the experiments of Loewi in 1921 it is likely 
that the effects observed were due to released epineph- 
rine, for which good evidence was obtained later (84, 
124). There is no evidence, however, that epineph- 
rine serves as adrenergic nerve transmitter in any 
animal. In the frog the spleen contains chiefly nor- 
epinephrine (105), and it can not be excluded that 
the epinephrine released on sympathetic nerve stimu- 
lation originates from chromaffin cells and not from 
adrenergic nerve endings. In such a case the substance 
released from the heart (which lacks coronary vessels 
in the frog) is not a neurotransmitter proper and the 
mechanism involved would be analogous to the re- 
lease of epinephrine from the suprarenals on stimula- 
tion of its preganglionic nerves. 

Although the theory of Cannon and Rosenblueth 
concerning the two sympathins is chiefly of historical 
interest is may be briefly outlined here. [For a de- 
tailed discussion see Cannon & Rosenblueth (20), 
and Rosenblueth (113).] According to this theory 
epinephrine is the adrenergic nerve transmitter, which 
on reaching the target cells combines with some 




20 25 30 

NUMBER OF TUBE 

FIG. 6. Column chromatogram of extract of beef splenic 
nerves after adsorption on aluminium oxide and elution, show- 
ing a maximum for norepinephrine. [From von Euler & 
Lishajko (132).] 



220 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



TABLE I . Norepinephrine Content of Beef Nervous Tissue 

in tig per gm (129) 



Splenic nerve 


8.5-,8.5 


Splanchnic ner\e 


4 


Sympathetic chain, thoracic 


■! •5-4-9 


Sympathetic chain, cervical 


0.6 


Mesenteric nerve 


1-5-3 


Superior cervical ganglion 


I 


Saphenous nerve 


0.2-1 


Phrenic nerve 


0.15-0.25 


Vagus nerve 


0. 1 


Spinal cord 


0. T 


Brain 


. 04-0 . 20 



cell constituent to form what was termed inhibitory 
(I) or excitatory (E) sympathin or both. These find- 
ings are readily explained on the assumption that 
the actions observed were either due to the true 
adrenergic neurotransmitter, norepinephrine, or to 
epinephrine released from other sources, presumably 
chromaffin cells, or a mixture of both, as suggested 
i)y Bacq in 1934 and subsequently proved by the 
demonstration of both amines in autonomically in- 
nervated organs (126). The term sympathin should 
preferably be abandoned in the physiological litera- 
ture since it does not discriminate between the neuro- 
transmitter and the hormones released as a result of 
preganglionic stimulation of chromaffin cells. 



Occurrence, Biosynthesis and Storage oj Adrenergic 
Nerve Transmitter 

Unless it is assumed that the chemical transmitters 
are being formed and released at the moment of 
nervous excitation it must be concluded that they 
are present in the axon and relea.sed from .some kind 
of store. Systematic studies of the content of trans- 
mitter substances in postganglionic nerves have been 
made both for the cholinergic and for the adrenergic 
system. Such experiments have shown that the content 
of norepinephrine in a nerve correlates well with the 
number of unmyelinated fibers of autonomic origin 
(i 1 1). As seen in table i the amount of norepineph- 
rine varies greatly and is highest in the splenic nerves 
which are known to contain practically only post- 
ganglionic sympathetic fibers. In other nerves, such 
as the vagus, the amount is quite small and this is 
true also for most motor nerves and the majority of 
sensory nerves. For technical reasons it is impossible 
to prepare nerves in their most peripheral parts, 
hence it has not been possible to study directly the 
content of the transmitters in the immediate vicinitv 



of the target cell, which for many reasons would 
have been desirable. On the other hand it has been 
possible partly to overcome this difficulty by making 
extracts of whole organs and estimating their trans- 
mitter content, thus measuring the total amount 
present in the tissue including the finest nerve rami- 
fications (iio, 129). Proof that the transmitter sub- 
stances so found are actually due to the presence of 
postganglionic nerve fibers has been obtained by 
studying the effect of denervation. If the postgangli- 
onic nerves are severed and allowed to degenerate, 
the amount of norepinephrine in the peripheral ti.ssue 
falls to very low figures or disappears completely. 
This indicates that the tissue is not able to store the 
transmitter by itself but does so by means of its 
autonomic nerve fibers. Further support for this 
opinion is provided by experiments showing that 
some 4 to 8 weeks after degeneration of the cardiac 
nerves the content of adrenergic transmitter in the 
sheep heart increases again and after the lapse of a 
few months reaches the original value (fig. 7) (55). 
.Similar results have been oljtained for other organs 



2P 






1,8 




D left cerv. symp removed 


^1.5 


ao 


ffl right " " 


5l> 




e lei! stellate removed 


21,2 




9 right " 

■ 


%\o 




%Ofi 


fB(nofmal) m 




iO!& 




■ 


|0,4 




■ 


0.2 



•% 


■ 
■ 


1 2 


3 4 5 6 






lime-weeks after last operation 



FIG. 7. Norepinephrine content of sheep hearts before and 
various times after svmpathetic denervation. [From Goodall 

(55)-] 



T.\BLE 2. Norepinephrine Content in Beef Organs 

in fig per gm (129) 



Spleen 

Lymph glands 

Heart 

Ciliary body and iris 

Liver 

Arteries and veins 

Lung 

Intestine 

Uterus 

Testicle 

Skeletal muscle 

Bone marrow 



1-5-3-5 

0.5-0.8 

o . 3-0 . 6 

0.4 

o . 25 

o. I-I 

0.15 

0.15 
0.15 

0.04 
0.04 
0.0 



AUTONOMIC NEUROEFFECTOR TRANSMISSION 



221 



such as the spleen and the kidney of the sheep (129). 
The stores of the transmitter substance can thus be 
estimated by extracting the tissue and subjecting it to 
chemical or biological analysis. The content of 
adrenergic transmitter in an organ (table 2) provides 
a measure of the relative supply of adrenergic nerves. 
Norepinephrine was first suggested as a link in the 
biosynthesis chain leading to epinephrine by Blaschko 
(11). The basis for this was given by Holtz, Heise & 
Liidtke (68) who discovered an enzyme capable of 
decarboxylating levo-dihydroxyphenylalanine (dopa) 
to its corresponding amine, hydroxytyramine (dopa- 
mine). This enzyme was present in liver and kidney 
and has also been demonstrated in the adrenals and 
in adrenergic nerves (69). While it has been shown 
experimentally that homogenates of the adrenal gland 
synthesize norepinephrine from tyrosine (74), via 
dopa (33) and dopamine (59), this sequence has not 
been shown for adrenergic nerves although there 
can be little doubt that this is the case. At any rate 
it has been found that extracts of the spleen or 
splenic nerves contain relatively large amounts of 
dopamine (117, 132). The biosynthesis may there- 
fore be depicted by the following .scheme: 



OH 



OH 



iOH 



CH2CHCOOH 



CH.,CHCOOH 



NH., 








NH., 


Tyrosine 




Dopa 







H 
OH 








A 


H 
OH 


CH.,CH2- 


NH2 


CHrCHOHNH 


D( 


jpamine 






No 


repinephrine 



It appears likely that the biosynthesis is located in 
the place of storage (see below). Analysis of extracts 
of autonomic nerves have shown that the norepineph- 
rine content is a function of the proportion of adrener- 
gic fibers. These contain the transmitter along their 
whole length and also in the jxU soma. A very 
marked accumulation in the terminal parts must be 
assumed for the following reasons. Splenic nerves of 
the beef contain about 15 ng norepinephrine per gm 
fresh tissue after removal of the sheath, while the 
content of the whole organ is about 3 //g per gm. Since 



all of the splenic norepinephrine disappears on section 
and degeneration of the adrenergic nerves to the 
organ it is assumed that the norepinephrine found in 
the organ is bound to its nerves. On the other hand 
it is inconceivable that 20 per cent of the splenic 
tissue consists of nerves, and it follows from this that 
some parts of the nerves, presumably the endings, 
contain much more of the transmitter than the main 
nerve trunks. 

Even afte r rern oval from the body, organs retaiin 
their adrenergic transmitter substance for a con- 
siderable time. A beef spleen may thus be stored at 
room temperature for 24 hours without any detectable 
loss of norepinephrine. This indicates that it is not 
present in a freely diffusible form and strongly sug- 
gests that it is bound in such a way as to prevent con- 
tact with inactivating enzymes. 

Evidence has been obtained for the storage of the 
hormones of chromaffin cells in specific granules (12, 
63). By increasing the acidity of the surrounding solu- 
tion to pH5 or lower, the chromaffin cell hormones 
are released from the granules (63). When the same 
principle was applied to the isolated spleen by per- 
fusi^ng it with a .solution containing acids such as as- 
corbic, citric or lactic acid, the transmitter substance 
was released and could be demonstrated in the per- 
fusion fluid (40). Also other substances which have 
been found effective in releasing the hormones from 
isolated granules had a similar action on the perfused 
spleen, such as detergents, digitonin and lecithinase 
from snake venom. 

These e.xperiments add support to the hypothesis 
(127) that the neurotransmitter is stored, and proba- 
bly manufactured, in specific structures in the 
adrenergic axon. Experiments by Euler & Hillarp 
(131) have demonstrated that a microgranular frac- 
tion rich in norepinephrine can be separated by high 
speed centrifugation from a homogenate of beef 
splenic nerves. The chemotransmitter is apparently 
stored in elements surrounded by a membrane since 
a suspension of the sediment in Ringer's solution 
does not give off norepinephrine to the surrounding 
fluid. If acid is added to PH4 in the suspension, the 
norepinephrine is instantaneously released, however, 
and can be demonstrated by ijiological and chemical 
methods in the suspension fluid. The micrograniilar 
stores are apparently specific for the chemotrans- 
mitter since the histamine which is abundant in the 
beef splenic nerves (about 100 ixg per gm nerve) is not 
present in the same structural elements. Certain 
cellular fractions have been found to contain more 
than 1.5 fig norepinephrine per mg dry weight or 



222 



HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



around 20 times the amount per mg dry weight found 
in the whole nerve before homogenization. 

The theory may then be advanced that the adrener- 
gic nerve transmitter is bound to elements which in 
principle are of a kind similar to those found in the 
chromaffin cells. Since these can be regarded as 
homologues of the postganglionic neurons it might 
be expected that their constituents with specific activ- 
ity are stored in a similar way. The structural ele- 
ments serving as stores may also well be the units for 
bio.synthesis. Apparently this takes place very rapidly 
so as to maintain a practically constant store. Con- 
tinuous and prolonged stimulation of nerves in vitro 
(88) or in vivo (129) does not deplete the stores. There 
is no evidence that the granules of the chromaffin 
cells leave the cell body in connection with the re- 
lease of the hormones; this may be assumed also for 
the storing elements of the postganglionic adrenergic 
neurons. It may be postulated that the microstruc- 
tures elaborating and containing the neurotrans- 
mitter are formed in the cell soma and transported 
along the axon towards the periphery by the axo- 
plasm flow (135). These assumptions would provide 
a satisfactory explanation for the findings that a) the 
chemotransmitter is accumulated in the terminal parts 
of the neuron, and that h~) continuous stimulation does 
not deplete the nerves of its chemotransmitter. The 
theory involving the assumption of intra-axonal 
microstructural elements thus seems to explain 
several phenomena encountered in the field of neuro- 
transmission. 

Release 

Stimulation of the adrenergic nerves, either directly 
or reflexly, immediately releases norepinephrine 
which is then allowed to diffuse to the adjacent tar- 
get cells. From the above section it may be inferred 
that the transmitter is released from microstructures 
and accumulated at the terminal parts of the nerves, 
presumably in a way similar to that operating in the 
chromaffin cells. The large number of discrete 
terminal ramifications ofl"er only short diffusion dis- 
tances, enabling each cell to be reached by the chemi- 
cal transmitter in a very short time. While under 
normal conditions the adrenergic chemotransmitter 
is released chiefly, if not entirely, as a result of reflex 
stimulation, various experimental approaches have 
been made in order to study the release in more 
detail, such as a) observation of the effects of direct 
nerve stimulation on the innervated organ. A) record- 
ing of the effects of stimulation of adrenergic nerves 



on remote organs, c) quantitatixe estimation of the 
content of the neurotransmitter in the venous effluent 
from the stimulated organ, and (T) measuring the 
release of transmitter from isolated nerves stimulated 
in vitro, or from organs perfused in vitro. 

INFLUENCE OF STIMUL.XTION FREQUENCY. The effect of 

Stimulation of the adrenergic nerves — or usually 
mixed nerves containing adrenergic fibers — provides 
the basis for most of our knowledge of the action of 
the adrenergic system on various target organs. A 
study of these effects not only permits qualitative in- 
formation on the type of effect on the organ but also 
offers opportunities for gaining quantitative infor- 
mation, for instance about the influence of stimulus 
strength and frequency on the effect. In this way the 
relea.se mechanism can be studied at least on a semi- 
quantitative basis which can hardly be accom- 
plished by reflex stimulation. 

While the technique of studying the response of an 
organ to variation in the intensity of the stimulus 
gives an idea of the excitability of the nerve fibers, 
information about the release mechanism is better 
obtained by varying the stimulus frequency. Such 
experiments are preferably performed u.sing stimu- 
lation intensities which will allow participation of 
all fibers. As shown in figure 8, the curves obtained 
by Rosenblueth (112) showing the relationship of 
stimulus frequency and effect on various autonomic 
effectors have the general shape of rectangular 
hyperbolas. The results show the noteworthy feature 
that considerable effects are achieved even at very 
low frequencies. As can be seen from figure 8, e\en 
frequencies of less than i per sec. are capable of 
causing marked effects. Nearly maximal actions have 
been recorded with frequencies of the order of 5 per 
sec, for instance on the piloerectors and the nictitat- 
ing membrane. The results imply that even very low 
frequencies are sufficient to release considerable 
amounts of the transmitter. In table 3 the optimum 
frequencies for a numjjer of effectors are given. Maxi- 
mal effects are obtained with frequencies varying 
from 20 to 30 per sec. in most effector systems. Even 
a frequency of 10 per sec. generally elicits more than 
80 per cent of the maximal response. 

.\n interesting difference is noted between the 
ratio of the effects of single stimuli and those of 
maximal tetanic stimuli on smooth and skeletal mus- 
cles, no doubt depending on the trigger mechanism in 
the latter. Thus the ratio between the effects is much 
higher for the smooth muscle than for the skeletal 
muscle. 



AUTONOMIC NEUROEFFECTOR TRANSMISSION 



223 




FIG. 8. Frequency-response curves of sympathetic effectors. .-1: abscissae, frequencies of stimulation 
of tlie lumbar sympathetics; ordinates, angles of erection of a hair in the tail of a cat. B: abscissae, 
frequencies of stimulation of the cervical sympathetic; ordinates, heights of the records of isotonic 
contractions of the nictitating membrane 15 sec. after the beginning of stimulation. C: as in B, but 
isometric contractions of the nictitating membrane. D : abscissae, frequencies of stimulation of the 
right cardioaccelerator nerves; ordinates, maximal increases of heart rate per 15 sec. [From Rosen- 
blueth (112).] 



Even after cutting a considerable portion of the 
nerve the maximal effect may be approached, pro- 
vided the frequency of stimulation is increased suffi- 
ciently. The effect of low frequencies on a partially 
severed nerve is smaller than in the intact nerve, 
however, which might be expected. 

The conclusion drawn from these experiments is 
that the neurotransmitter diffuses to the neighboring 
cells as its concentration is raised by increasing the 
stimulation frequency. The principle of activation of 
smooth muscle cells may therefore be a general re- 
lease of transmitter within the mass of these cells, 
rnaking individual innervation as for the skeletal 
muscle fibers unnecessary. 



T.'VBLE 3. Frequencies of Preganglionic Stimulation, 
Giving Maximal Response of Effectors (20) 



Effectors 
Sympathetic 

Pilomotors 

Nictitating membrane 

Pregnant uterus 

Intestine 

Adrenal medulla 

Heart (postgangl.) 
Parasympathetic 

Heart 

Submaxillary gland 

Stomach 



Frequency 
Stim. per sec. 



■5 
20 
20 
20 
25 
25 



30 
35 
25 



224 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



MAX CONSTRICTION 
100 




BASAL 
BLOOO FLOW 

PHYSIOL DISCHARGE RANGE 



JSTIM 
30 FREQ 



FIG. 9- Vasoconstrictor effect of electric stimulation of lumbar sympathetics at \arying frequencies 
in the cat. Striped area indicates the variations obser\'ed in 40 experiments. A represents a\'erage of 
10 experiments with the biggest response; B, average response after vasodilator drugs. [From Folkow 
(46).] 



The effect of stimulation at various frequencies of 
sympathetic nerves to the muscular Ijlood vessels in 
the lower part of the hind limlj of the cat has been 
measured by recording the outflow (46). 

Figure 9 shows the correlation between stimula- 
tion rate and the constrictor response. It is clearly 
seen that low stimulation frequencies are very effec- 
tive. This applies also to cutaneous blood vessels (22). 

A detailed analysis of the mechanism of the release 
has been made by Brown & Gillespie (14) using the 
cat's spleen. Samples of venous blood were collected 
and the norepinephrine content assayed on the ar- 
terial pressure of the pithed rat. Supramaximal 
stimuli were applied to the splenic nerve, the total 
number of stimuli being 200, irrespective of the fre- 
quencv. Both adrenal glands were removed and the 
splanchnic nerves cut. The output of norepinephrine 
was expressed as amount released per stimulus. 

As illustrated in figure io.4 the norepinephrine out- 
put per stimulus was low at low frequencies, but as 
the frequency increased the amount found in venous 
blood rose sharply to a maximum at about 30 stimu- 
lations per sec. Since the output per stimulus was 
the same before and after addition of isopropyl 
isonicotinyl hydrazine (Marsilid), an effect of amine 
oxidase on the transmitter liberated at lower frequen- 
cies could be excluded. The possibility was also dis- 
cussed that, although the amount of transmitter re- 
leased by each nerve volley might be constant, more 
was 'utilized' by tissue receptors at a low rate of stim- 



ulation. After blocking tissue receptors with N-N- 
dibenzyl-/3-chloroethylamine (dibenamine), it was 
found that the output per impulse reaching the blood 
was greatly increased at the lower frequencies and 
maintained a constant value at different frequencies. 
From these observations it was concluded that the 
norepinephrine release per nerve volley is constant 
and that the fraction removed by the tissues is 
greater at the lower frequencies of stimulation (cf. 
section on removal of transmitter, p. 227). 

The experiments quoted above may have an in- 
teresting implication in that the small or absent over- 
flow at low stimulation frequency (or adrenergic 
nerve activity) and the larger overflow at higher ac- 
tivity may cause an excretion pattern in the urine 
which 'amplifies' the actual release and makes differ- 
ences more pronounced than would be expected from 
the activity of the effector. 

EFFECTS ON REMOTE ORGANS. This method of study- 
ing the release of the adrenergic transmitter is the 
one which led to the discovery and demonstration of 
such a mechanism. The first experiments of this kind 
were made by Cannon & Uridil in 1921 (21) who ob- 
served the effect of stimulating the li\cr nerves on the 
heart and iris .sensitized by denervation. They as- 
cribed the effect to a "special and unknown sub- 
stance" apparently being set free by the stimulation. 
This kind of experiment was developed further by 
Cannon and Rosenblueth and their co-workers in the 



AUTONOMIC NEUROEFFECTOR TRANSMISSION 



12 



0-8 



4 




Frequency (stimuli/eec.) 



J I I L 



I I I I I 




12 - 



0-8 



20 



40 



60 



80 100 



200 800 



4 



Frequency (per sec.) 



10 



2U 



30 



40 



50 



FIG. lo. --1; Mean output per stimulus of 'sympathin' plotted 
against the frequency of stimulation. At all frequencies of 
stimulation the total number of pulses was 2oq. The vertical 
lines represent the standard errors of the means. Figures for 
loo, 200 and 300 pulses per sec. are single observations. B: 
First part of the graph in A with an extended scale for frequency. 
The individual results from three animals previously given 
dibenamine are shown. The output per stimulus at 10 pulses 
per sec. has increased and equals the maximum in the un- 
treated animal. There is no obvious variation with frequency. 
[From Brown & Gillespie (14).] 



work on 'sympathin'. While the study of the trans- 
mitter release in this manner, by recording the effect 
on sensitized remote target organs, was valuable in 
the elucidation of the transmission mechanism as 
such, its physiological significance is doubtful. Even 
though Cannon and Rosenblueth and their co- 
workers obtained increases in heart rate, dilatation of 
the pupil and contraction of the nictitating membrane 
in denervated organs after stimulation of sympathetic 
nerves in other parts of the body, the appearance of 
remote effects caused by transportation of the re- 
leased transmitter by the blood is by no means a 
constant phenomenon. 



The failure of some authors (22) to observe remote 
effects even on the highly sensitized denervated 
nictitating membrane in spite of intense stimulation 
of sympathetic nerves has been taken to indicate the 
presence of peripheral inactivation mechanisms which 
largely eliminate an overflow of transmitter. How- 
ever, a physiologically occurring overflow in the 
meaning of Cannon and Rosenblueth cannot be 
denied for the following reason. If the catechol amines 
are estimated in urine from adrenalectomized pa- 
tients the amounts of epinephrine are very low while 
the norepinephrine content tends to be even higher 
than in normal subjects (129). The only possibility 
for norepinephrine to occur in the urine then is a 
release from some source in the body other than the 
adrenals. Since the adrenergic nerves are known to 
contain large amounts of this transmitter, it appears 
legitimate to assume that during the incessant ac- 
tivity of the adrenergic system a certain overflow of 
transmitter takes place continuously. 

As to the value of the remote effects studied by 
Cannon and Rosenblueth as a proof of chemotrans- 
mission from nerves, it should be borne in mind that 
nervous stimulation might also cause a release from 
chrornaffin cells present in the tissues. This criticism 
does not invalidate their conclusions in principle 
since there is good evidence in some of Cannon and 
Rosenblueth's experiments that at least some of the 
effects are due to the release of norepinephrine. 

It is interesting to note that the so-called inhibitory 
sympathin is obtained when the splanchnics are 
stimulated but not when the hepatic nerves are stimu- 
lated (fig. 1 1). It is known that the s£lanchnic_nerves 
may innervate groups of chromaitiin-cells^at various 
sites. Their secretory products may then be carried 
by the blood stream to excite the denervated organ. 
In case of the hepatic nerves there was only a stim- 
ulating effect but no inhibitory effect on the dener- 
vated uterus of the cat, indicating that practically only 
nore£ine£hrine was released in this case. As far as 
can be ascertained at the present time this norepi- 
nephrine is released from adrenergic nerve endings. 

The question whether reflex liberation of the 
adrenergic transmitter could be large enough to 
cause actions on remote organs has al.so been studied 
(80). As a result of afferent sciatic or brachial nerve 
stimulation it was possible to demonstrate a contrac- 
tion of the denervated nictitating membrane in the 
adrenalectomized cat. It has also been possible to 
show a reflex liberation of the adrenergic transmitter 
by action on remote organs, for instance after excite- 
ment and struggle (103). The slower development of 



■226 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 




FIG. 1 1 . Upper curves, decentralized nictitating membrane; 
■lower curves, denervated nonpregnant uterus of the cat. Con- 
traction upwards. Time, 30 sec. A: hepatic nerves stimulated. 
B: right splanchnic nerves stimulated after exclusion of ad- 
renals. C: same as in B and injection of 1.5 Mg epinephrine. 
D: duodenohepatic nerves stimulated. E: same as in D after 
severance of duodenal nerves. [From Cannon & Rosenblueth 
(19)-] 



the blood-borne action on the denervated heart after 
reflex excitation, resuhing from struggle, as compared 
with the rapid and large effect in cases where the 
adrenals were active may be explained by the gradual 
and prolonged release of moderate amounts of trans- 
mitter in the former case. The activation of the 
adrenals has a tendency to cause an 'explosive' re- 
lease. A continuous liberation into the blood stream 
during 'sham rage' has also been noted (136) in 
decorticate cats showing a quasiemotional state as 
exidenced by the reduction in the rate of the dener- 
vated heart after section of the hepatic nerves. Even 
as a consequence of normal emotions a release of 
transmitter into the blood has been observed. 

On exposure to cold no sign of continuous effect on 
the denervated nictitating membrane of the cat was 
found, however (107). The persistent erection of hairs 
when the animal is in cold surroundings is apparently 
not accompanied by a liberation of enough trans- 
mitter to affect remote organs even if these are sensi- 
tized. 

During^ypoglycemia there is no evidence for ac- 
tivation of the sympathetic system as a whole. Only 
a selective release of epinephrine from the suprarenal 
has been demonstrated by direct analysis of the 
venous blood (38). The contention expressed by Can- 
non and Rosenblueth that "it is characteristic of the 



sympathetic system, when specially excited, to act as 
a whole; thus adrenine is secreted by splanchnic im- 
pulses at the same time that sympathetic impulses 
elsewhere in the body are liberating sympathin" has 
not been corroborated by later experiments and 
experience. It is now reasonably certain that the 
secretion of epinephrine is a process which occurs in- 
dependently and often during quite other conditions 
than the activation of other parts of the sympathetic 
system. It would also appear peculiar if the action of 
epinephrine in maintaining blood sugar homeostasis 
should be obligatorily linked with, for instance, a rise 
of arterial pressure as a result of generalized adrener- 
gic activity. The statement of Cannon and Rosen- 
blueth that "adrenine and sympathin collaborate in 
affecting structures innerv-ated by sympathetic 
nerves" is only true in a restricted sense and its biolog- 
ical significance is too limited to be set forth as a gen- 
eral rule. The statement also illustrates the hazards 
of using the term 'sympathin' since this may repre- 
sent either epinephrine or norepinephrine. It may be 
recalled that the two amines have opposite effects for 
instance on the vessels of the skeletal muscles (i , 8, 37). 
Even if the leakage of transmitter into the blood stream 
is negligible from the point of view of physiological 
action, this phenomenon has been of great heuristic 
value as in Cannon and Rosenblueth's work and also 
in the extensive work dealing with the excretion of 
the neurotransmitter in urine (67, 129). 

Information about the nerv-e transmitter may also 
be gained by collecting blood or perfusing fluid from 
an organ during stimulation of the sympathetic 
nerves, and by recording the effects of this fluid on 
suitable test organs. Studies of this kind are in prin- 
ciple similar to the pioneer experiments by Loewi. 
Active substances in the effluent have been demon- 
strated in many instances, such as from the frog's 
stomach (13), the aqueous humour (3), the rabbit's 
intestine (45) and the dog's tongue (6). 

By the use of an appropriate testing technique it 
could be shown later that the active substance re- 
leased by adrenergic nerve stimulation conformed 
in its properties with norepinephrine (14, 93, 98, 106, 
108). In these studies the venous plasma of the stimu- 
lated organ was tested. Most investigators have also 
stated that s maller amoujits of epinephrine were 
sometimes also liberated. The significance of the 
simultaneous appearance of small amounts of epi- 
nephrine will be considered below. 

The release of epinephrine-like materials on stimu- 
lation of the vagus nerve to the atropinized heart has 
also been reported (65, 94). The former authors con- 



AUTONOMIC NEUROEFFECTOR TRANSMISSION 



227 



eluded that the epinephrine-Hke substance was re- 
leased from intracardiac adrenergic neurons con- 
trolled bv preganglionic fibers in the vagus. Whether 
the substance was released from neurons proper or 
from chromaffin cells is not clear, however. 

STIMULATION OF ISOLATED NERVES. Attempts havc been 
made to study the release of the adrenergic trans- 
mitter by stimulating isolated nerves, thus avoiding 
the possibility of interaction of the inner\'ated tissues. 
In unpublished experiments Gaddum & Khayyal 
(50) stimulated an isolated sympathetic nerve sus- 
pended in salt solution and found that a sympathomi- 
metic substance was released into the solution. This 
efTect was later attributed to damage to the nerve by 
the stimulating electrodes (53). However, the original 
finding was later confirined (79). This is in agreement 
with the fact that the whole nerve trunk contains 
norepinephrine. 

EXHAUSTIBILITY. Studies on the exhaustibility of the 
transmitter sources have shown that even prolonged 
stimulation, reflex or direct, does not seem to lessen 
the release. Orias (104) stimulated the preganglionic 
fibers of the cervical sympathetic 10 times a sec. for 
I hour and found no .signs of fatigue in the responses 
of the nictitating membrane. These experiments were 
repeated by Dye (39) who applied not less than 
108,000 Stimuli during 3 hours to the preganglionic 
nerves without evidence of exhaustion. Luco & Goni 
(88) found that stimulation of sympathetic nerves 
for I hour did not diminish the content of transmitter 
in the nerve. It may therefore be assumed that release 
of the transmitter can continue for an unlimited time. 
This is an indication in the first place that the trans- 
mitter is readily resynthesized but also that the 
release mechanism is built to render continuous 
service. 

Removal of Transmitter 

Although it is apparent from the observations of 
remote effects of adrenergic nerve stimulation and 
from the excretion of norepinephrine in urine that a 
certain proportion of the released neurotransmitter 
is transferred into the circulating blood, it is generally 
assumed that most of the transmitter is being inacti- 
vated at or near the site of release (16, 47}. 

The experiments of Brown & Gillespie (14) indicate 
that the removal of the transmitter is more efficient 
when it is released at a slow rate. As to the mechanism 
of removal, their experiments suggest that the trans- 



mitter is being attached to a certain extent to the 
effector cells and presumably inactivated at this site. 

Our knowledge about the mechanism of inactiva- 
tion is still very incoinplete. The inability of iso- 
propyl isonicotinyl hydrazine (Marsilid) to affect to 
any noticeable extent the amount of transmitter re- 
covered in the effluent blood after stimulation of the 
splenic nerves does not support the common opinion 
that amine oxiclase pla ys an importan t part in this 
respect. 

In experiments in which the transmitter was re- 
leased from a perfused spleen by various chemical 
means, the amount of norepinephrine found in the 
effluent was not greatly influenced by adding amine 
oxidase inhibitors to the perfusion fluid (129). More- 
over, administration of Marsilid to an animal does 
not augment the degree or duration of adrenergic 
reflex actions in the cat, such as the pressor eflfect of 
carotid occlusion, indicating that amine oxidase, at 
any rate, does not attack the transmitter between the 
moment of release and the action on the efTector cell. 

The problem of the removal of the transmitter 
after its release may be regarded from two aspects. 
One part of the transmitter apparently is d]rectly 
at_tached to the effector cells [or 'utilized' (14)] while 
another portion is leaking into the blood vessels, or 
by-passing the target cells as it were. It is conceivable 
that after saturation of the target cells the remainder 
of the released transmitter diffuses through the capil- 
lary wall and enters the blood stream. The situation 
might be regarded as analogous to that prevailing 
during reabsorption of a threshold substance by the 
renal tubules where an excess causes an ' overflow' 
into the urine. If the amount of the transmitter which 
is caught by the effector cells is considered first, it 
appears probable that it is being inactivated by some 
process so far unknown. It may well be that on many 
occasions this part represents the greatest part of the 
released transmitter. The second part which is not 
taken up by the cells may theoretically be attacked 
by enzymes on its diffusion way to the blood or 
lymph capillaries. Apparently this is not the case since 
amine oxidase inhibitors did not appreciably alter 
the yield in the effluent Ijlood (14). Not even after 
having reached the blood stream is the inactivation 
complete as seen by the excretion in urine of neuro- 
transmitter which undoubtedly originates in adre- 
nergic nerves, as indicated by the excretion in 
adrenalectomized patients. Knowing the proportion 
of norepinephrine e.xcreted in urine after intravenous 
infusion at a constant rate, it seems po.ssible to obtain 
an idea of the ' overflow' of adrenergic transmitter 



228 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



per unit of time. From infusion experiments in m^n 
it has been found that the proportion of norepi- 
nephrine excreted in urine is 1.5 to 4 per cent of that 
infused durinc; the same time (129). If the total ex- 
cretion of norepinephrine (free and conjugated) in 
man during 24 hours, when the subject is performing 
daily routine work but no severe muscular work, can 
be estimated at 60 fxg (109, 129), the amount of 
neurotransmitter overflow may be estimated at ap- 
proximately 2 to 3 mg per 24 hours. 

The careful study of the distribution of monoamine 
oxidase (MAO) in various nerve cells by Koelle & 
Valk (76) does not support the opinion that MAO is 
specifically occurring in adrenergic nerves, since no 
significant differences were found in the MAO ac- 
tivity in nerve cell bodies and fibers of the stellate, 
superior cervical, nodose, dorsal root and ciliary 
ganglia of the cat. The enzyme is localized in smooth 
muscle cells of blood vessels. It is absent in cardiac 
muscle, but high activity is found in renal tubule 
cells and hepatic cells. Since the removal of the trans- 
mitter by inactivating enzymes is more likely to occur 
in the target cells than in the neurons producing the 
transmitter this result is not unexpected. 

Small amounts of the transmitter are successfully 
removed during the passage of blood through the 
tissue, up to 90 per cent during a single passage 
through muscle and skin. This is in harmony with 
the findings that after infusion of norepinephrine and 
epinephrine in man only a small percentage appears 
in the urine, the rest being inactivated. 

Mechanisms of inactivation other than by MAO 
are conceivable, such as by catechol oxidases and 
peroxidases and by conjugation. The relative unim- 
portance of the inactivation of circulating catechol 
amines by MAO is further borne out by the observa- 
tion that cobefrine (a-methyl-rfZ-norepinephrine) is 
excreted in a similar small proportion as epinephrine 
and norepinephrine after injection in man (129), 
although it is not attacked by this enzyme. It must 
therefore have been inactivated (to more than 90 per 
cent) by some other mechanism which presumably 
would have been similarly active on the catechol 
hormones. 



POSSIBLE ADRENERGIC NERVE TRANSMITTERS 
OTHER THAN NOREPINEPHRINE 

It may well be asked whether there is any way of 
distinguishing between the release of the chemotrans- 



mitter from the nerve terminals and the secretory 
products from chromaffin cells in the tissues. Since 
very little is known about the mass and distribution 
of such scattered chromaffin cells or whether they 
secrete epinephrine or norepinephrine or both (and 
in the latter case the relative proportions), it is hard 
to evaluate the amount of neurotransmitter sensu 
strictwn which is released upon stimulation of svinpa- 
thetic nerves. Assuming that chromaffin cells are 
present in an organ, they would be made to release 
their secretory products by stimulation of the pre- 
ganglionic fil:)ers in the sympathetic nerve. 

A partial answer to this problem has been afTorded 
by studies on the content of the active catechol 
amines in tissues and organs. There is good evidence 
that the catechol amines found in extracts of organs 
and tissues are derived from their adrenergic nerves 
and chromaffin cells. This is shown by a) the large 
reduction or disappearance of the catechol amines 
after postganglionic denervation (18, 55, 129), h) the 
absence of these suijstances in the nerve-free placenta 
(i 16, 124) and c) the reappearance of such substances 
upon regeneration of the postganglionic nerves 
(55, 129). It is known that section and degeneration 
of the preganglionic fibers that innervate the chro- 
maffin cells do not cause depletion of the secretory 
products of these cells, while section of the postgangli- 
onic fibers causes disappearance of their transmitter 
substance. It is thus possible by analysis of the cate- 
chol amine content of an organ after preganglionic 
and postganglionic denervation to obtain information 
on the occurrence of chromaffin cells. The results of 
such experiments liave been that ' postganglionic' 
nerve section usually leaves a small remnant of 
activity. It is typical of this that the proportion of 
epinephrine is higher than it is in the organ with its 
nerves intact (55, 129). Sometimes the epinephrine 
content is unchanged. The conclusion has been 
drawn from these experiments that_gractic_ally all of 
the norepinephrine_is present in the postganglionic 
nerves while the epinephrine must have been located 
outside the adrenergic neurons, in all likelihood in 
chrouKiffin^cells. Such cells have been described in the 
heart b\ Trinci (123). 

Further evidence along the same line has been ob- 
tained from experiments on the isolated perfused 
rabbit heart either beating spontaneously or dri\'en 
electrically at a faster rate (32). By recirculation of 
the perfusing fluid it is possible to concentrate the 
active substances released from the heart. After sepa- 



AUTONOMIC NEUROEFFECTOR TRANSMISSION 



229 



ration by chromatography and biological estimation 
on the rat's arterial pressure the following results 
(expressed as micrograms per heart in 40 min.) were 
obtained. 

Electrically drisen 
Xorepinephrine Epinephrine 

Mean: 0.01 ± o.oi 0.08 ± 0.02 

Spontaneously beating 
Mean: 0.02 ± 0.0 1 0.08 ± 0.02 

These results are of interest since they clearly show 
that the proportion of epinephrine is far higher than 
that occurring in extracts of hearts or in the coronary 
blood plasma after stimulation of cardiac sympa- 
thetic nerves (129). The reason for the large release 
of epinephrine in the spontaneously beating heart is 
obscure, howe\'er. It therefore appears justified to 
conclude that the epinephrine released probably 
originates from chromaffin cells. On the other hand 
the norepinephrine left in an organ after postgangli- 
onic denervation constitutes such a small part of the 
total amount found in the organ with its nerves intact 
that the amount normally released on sympathetic 
nerve stimulation must come from the adrenergic 
nerves. Analysis of the urine from adrenalectomized 
patients has also shown that the amount of epineph- 
rine is exceedingly small compared with that of 
norepinephrine (129). Moreover, no increase in the 
epinephrine output was observ-ed in the adrenalecto- 
mized patients subjected to tilting head-up which 
doubled the norepinephrine output. This speaks 
strongly against the assumption that epinephrine is 
released from adrenergic nerves in man. Moreover, 
the epinephrine content of splenic nerves is as a rule 
extremely low, a fact .suggesting that the small epi- 
nephrine amounts found in spleen extracts (129) or 
sometimes in the effluent blood from the spleen after 
stimulation of its nerves (108) is not part of the 
neurotransmitter. For a discussion of the adrenoxine 
theory of Bacq & Heirman (5) the reader is referred 
to the survey on this subject by the same authors. 

It is clearly a matter of choice whether the epi- 
nephrine released from chromaffin cells in the tissue 
upon sympathetic nerve stimulation should be re- 
garded as a chemical transmitter. If one agrees to 
that terminology, the release of suprarenal medullary 
hormones should likewise be called chemical trans- 
mission. This, however, is apt to cause confusion of 
the concepts. It must still be left an open question 
whether the epinephrine-like actions observed upon 



stimulation of sympathetic fibers to the skin (57) are 
due to a release from chromaffin cells. 

The possibility of dopamine serving as a neurotrans- 
mitter requires further study. Holtz, Credner & 
Koepp (66) showed that it occurred normallv in 
urine. Its formation was explained as an action of 
/-dopadecarboxylase on dopa. Later dopamine was 
demonstrated by Goodall (55) in extracts of the 
suprarenal gland and in extracts of mammalian heart. 

.Since the presence of catechol amines in organs is 
correlated with their adrenergic nerves or chromaffin 
cells, it might be expected that the former aLso con- 
tain dopamine. This has been shown to be the case; 
dopamine was found in comparatively large amounts 
in extracts of splenic nerves (i 17). It seems reasonable 
to assume that the dopamine found in organs is pres- 
ent in their adrenergic nerves. If this assumption is 
correct the question arises as to how dopamine is 
stored and whether it is released upon nerve stimula- 
tion. Generally the amount of dopamine in an organ 
is hardly large enough to cause biological efifects 
comparable to those caused by the norepinephrine. 
However, the bovine lung contains large amounts of 
dopamine in comparison with norepinephrine (132), 
and it cannot be ruled out that dopamine exerts bio- 
logical actions in this case. After chromatographic 
separation the amount of dopamine was found by bio- 
logical and chemical methods to be 0.5 to i fig per 
gm tis.sue while the norepinephrine was o.oi to 0.03 
Mg per gm. Since the biological activity of the two 
substances is approximately in the proportion 50 to 
100: 1, it is obvious that dopamine may be biologicallv 
significant in the lung. 

It has been claimed that isopropylnorepinephrine 
occurs in small amounts in extracts of the adrenal 
gland (81). Apparently the amounts are too small to 
permit detection with the usual colorimetric and bio- 
logical methods, since these give very good agree- 
ment with the figures for epinephrine and norepi- 
nephrine. However, it has been reported that it can 
be separated by chromatographic technic, a certain 
fraction showing the characteristic biological action 
of the isopropyl compound. 

It has been reported that, after stimulation of the 
sympathetic nerves to the lungs, the isopropyl com- 
pound appears in the effluent blood (82). Chromato- 
graphic separation of catechol compounds in extracts 
of up to 1000 gm bovine lungs have failed to detect 
this fraction, although catechol acetic acid, dopamine 
and norepinephrine are readily identified (132). 



23° 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY 



THE CHOLINERGIC NERVE TRANSMITTER 

Identification 

Dixon & Hamill (36) pointed out as early as 1909 
that there was very little inherent difference between 
the action of muscarine on the heart and electrical 
excitation of the vagus. They continue: "If it is per- 
missible to argue from analogy there is reason in the 
suggestion that excitation of a nerve induces the local 
liberation of a hormone which causes specific activity 
by combination with some constituent by the end 
organ muscle or gland." Only a few years later 
Dale (25) and Dale & Ewins (30) related the phe- 
nomena observed on stimulation of parasympathetic 
nerves to some earlier research by Hunt & Taveau 
(71). Among a large series of choline esters prepared 
by them, acetylcholine was shown to be the most bio- 
logically active, on an average about 1000 times more 
active than choline. During studies on ergot extracts. 
Dale (26) found a substance which produced actions 
similar to muscarine and identified this substance 
with acetylcholine. In his further workJDale was able 
to state that the actions of vagus stimulation and also 
other actions of the cranial and sacral divisions of the 
autonomic system were mimicked very faithfully by 
ac_etylcholine. The effects were remarkably evanescent 
and were always abolished by a small dose of atro- 
pine. On the basis of these observations by Dale it 
became extremely likely that acetylcholine actually 
was the substance which causes the effect of para- 
sympathetic nerve impulses on the target cells. 
Further support for the idea that the substance re- 
leased at the parasympathetic nerve endings was 
acetylcholine was supplied by Dale & Dudley (29) 
who showed in 1929 that it was present in the spleen 
of the horse and the ox. They prepared the substance 
and isolated it as chloroplatinate. 

The identification of the parasympathetic cholin- 
ergic nerve transmitter is based upon biological tests. 
The amounts of acetylcholine which are liberated 
and occur in the organism are generally too small to 
be determined by chemical methods. Some of the bio- 
logical methods are very sensitive, but on the other 
hand the specificity is not always above doubt. The 
methods most widely used are the negative inotropic 
action of acetylcholine on the heart of the frog, the 
hypotensive effect in the cat and the contracting; effect 
on the intestine of the guinea pig or other animals. 
Other preparations which may yield more specific 
results are the leech muscle, the rectus abdominis 
muscle and the isolated lung of the frog. The isolated 



heart of the clam Venus mercenaria has also been used. 
For the identification of acetylcholine, the finding of 
Fiihner (49) that the dorsal muscle of the leech was 
greatly .sensitized to acetylcholine by addition of 
physostigmine was one of the more important. The 
preparation was introduced as a specific and quanti- 
tative biologic test for acetylcholine in 1932 by Minz 
(95). After preparation the muscle is suspended in 
Ringer's solution from one-half to several hours to 
relax it, and physostigmine is added to the solution in 
a concentration of 1-200,000 to 1-2,000,000. After 
about 20 min., the muscle is highly sensitized to 
acetylcholine so as to detect and measure acetylcho- 
line in concentrations as low as io~'. The frog rectus 
is less .sensitive but fairly specific for acetylcholine. 
The isolated frog lung has also been used and may 
have an even higher sensitivity than the leech muscle; 
it is claimed to contract in an acetylcholine solution 
of io~'* (34)- The heart of Venus mercenaria has also 
been reported to have high sensitivity to acetylcholine, 
up to io~^-, although it varies at different times of the 
year. 

In order to allow the conclusion that the actions 
observed on these test preparations actually have 
been due to acetylcholine, certain other conditions 
must be fulfilled. The action has to be increased by 
drugs inhibiting the acetylcholine esterase such as_ 
physostigmine, the activity should disappear after 
incubation with blood and the active principle should 
be inactivated when exposed to in alkali for 10 min. 
at room temperature, which is typical of choline 
esters. As a general rule different kinds of tests have 
to be consistent, i.e. when compared with a standard 
of acetylcholine the unknown extracts should elicit 
the same quantitative action in relation to acetyl- 
choline (fig. 12). 

One of the chief difficulties in demonstrating the 
neurochemical transmission from cholinergic nerves 
arises from the fact that in most cases the para- 
sympathetic nerves have their autonomic synapses 
very close to the target organ. Therefore, stimulation 
of the nerves also releases acetylcholine from the 
preganglionic nerve. The acetylcholine released by 
stimulation of vagus nerve in the frog's heart may 
actually be due partly to the release of the substance 
from the synapses. 

The introduction of physostigmine in experimental 
work made it possible to demonstrate the mediated 
effect with greater certainty since the substance was 
not immediately destroyed. Loewi's original experi- 
ments were later confirmed by many others. Among 
the sources of transmitter which have been tried mav 



AUTONOMIC NEUROEFFECTOR TRANSMISSION 23 1 




FIG. 12. Tests of a perfusate of physostigminized Locke's solution passing through the vessels of 
the stomach of a dog during vagal stimulation. The samples collected before stimulation were slightly, 
if at all, active. A: effects on the arterial pressure of a physostigminized cat under chloralose. B: 
isolated frog heart (Straub). C: physostigminized rectus abdominis of the frog. D : physostigminized 
leech muscle. In each series, B shows the effect of the perfusate collected during vagal stimulation ; 
A and C correspond to two strengths of acetylcholine (C is double A'). [From Dale & Feldberg 
(300 



be mentioned the heart and the sahvary glands 
(61, 134) in physostigminized animals. Particularly 
illuminating were the experiments by Feldberg & 
Krayer (44) who showed that blood from the coronary 
veins of physostigminized animals produced a con- 
traction of the leech muscle shortly after vagal stimu- 
lation. This effect was abolished by atropine and the 
active substance was destroyed by blood. Even re- 
flexly released transmitter was demonstrated in this 
way. 

Although the cholinergic transmitter has not been 
identified with the same certainty as the adrenergic 
one, the sum of evidence obtained by indirect methods 
leaves no serious doubt that it is either acetylcholine 
or some other choline ester with very similar action 

(15)- 

In the autonomic neurotransmission to the salivary 
glands both adrenergic and cholinergic fibers seem to 
take part. By studying the distribution of cholin- 
esterase Koelle (75) found in the cat, rabbit and 
rhesus monkey that the concentration of the true 
cholinesterase was higher in cholinergic neurons 
than in adrenergic and sensory neurons. Cholin- 



esterase was also found to form a fine network around 
the outside of the acini while it was not found in the 
acinar cells (118). The network is united with the 
nerve trunk and is considered to be cholinergic in the 
submaxillary gland and adrenergic in the sublingual 
gland. 

Occurrence, Biosynthesis and Storage 

It may be assumed that, if the postganglionic nerve 
endings release acetylcholine during nerve stimula- 
tion, this has been synthesized and stored in the axon. 
For this reason acetylcholine would be expected to 
occur as a natural constituent of cholinergic nerves. 
In this connection only the postganglionic fibers are 
being considered. Analysis of the acetylcholine con- 
tent of such fibers has shown large amounts in the 
short ciliary nerves, 3 to 8 /ng per gm, which is only 
a little less than the amounts found in motor nerve 
fibers or in preganglionic fibers (92, 125). The figures 
are much higher than the acetylcholine content in 
postganglionic sympathetic fibers, such as the splenic 
nerve where the acetvlcholine-like action onlv corre- 



232 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



sponds to 0.2 to 0.5 Mg per gni- The difFercnce in 
content suggests a specific function of the excess 
acetylcholine in the postganglionic parasympathetic 
fibers. The high content in these may be regarded as 
strong support of the idea that these fibers act by 
releasing acetylcholine. [For further details concern- 
ing the occurrence and biosynthesis of acetylcholine 
in cholinergic nerves see Burgen & Macintosh (15), 
Gaddum (50) and Rosenblueth (113).] 

The method of estimating the amount of acetylcho- 
line directly in the tissue cannot be used, however, to 
estimate the amount of the cholinergic postganglionic 
transmitter, since this will also occur in preganglionic 
autonomic fibers and in motor nerves and possibly 
also in small amounts in all kinds of nerves. 

The biosynthesis of the cholinergic transmitter 
has been l.ir^c K elucidated by the studies of 
Nachmansohn & Machado (102). These authors 
were ai^le to show that an extract from rat brain con- 
tained an enzyme system which could synthesize 
acetylcholine in the presence of ATP as the source of 
energy. This enzyme was called choline aceL)dase. It 
was shown later that the acetylcholine synthesis occurs 
in two steps. In a first reaction acetate is transformed 
into active acetate, and in a second the active acetate 
combines with choline to form acetylcholine (78). 
The research work of Stern & Ochoa ( 1 20) and others 
indicates that cholin e acetylase catalyzes the last step 
in the acetylcholine formation and that the aciise 
acetate is an acetyl coenzyme (coenzyme A). The 
acetate used for the synthesis has to be activated by 
inc. Ills of ATP, coenzyme and a transacetylase. The 
active acetate thus formed is used for the final synthe- 
sis of the acetylcholine. Choline acetylase has been 
extracted from brain and from electric organs but is 
also present in all nerve tissues. It has even been 
demonstrated in tissues from various invertebrates, 
such as annelids and flatworms. The presence of cho- 
line acetylase in mitochondrial fractions in homoge- 
nates of brain (60) suggests that this may be the case 
al.so in the postganglionic neurons. 

As to the storage of the cholinergic transmitter it 
appears likely that it is confined to structural elements 
as demonstrated for adrenergic nerves. Some indirect 
support for the opinion that acetylcholine is also in- 
closed in separate particles may be found in the early 
experiment by Loewi & Hellauer (86). Loewi (85) 
points to the finding that when nerve tissue is ex- 
tracted with Ringer's solution, the bulk of acetylcho- 
line is found in the insoluble residue but that the use 
of hypotonic .solution causes the greater part of the 
acetylcholine to be released. This suggests that the 



acetylcholine is located in particles surrounded ijy a 
membrane similar to mitochondria. When Ringer's 
solution is used for extracting the acetylcholine in a 
cholinergic nerve, such as the vagus, most of the 
acetylcholine goes into solution, however. It is also 
noteworthy that when acidified solutions are used, 
the total amount of acetylcholine is extracted as is 
also the case when extraction is made with acidified 
alcohol. Some of the acetylcholine may be i^ound to 
some lipid complex soluble in ether, which acetylcho- 
line in itself is not (86). 

An analogous i^ehavior is shown by epinephrine 
and norepinephrine and histamine. It therefore seems 
possible that these amines form ether-soluble but 
water-insoluble compounds in the particles. It is of 
interest in this connection that Hillarp & Nilson (64) 
found a high content of phosphatides in the supra- 
renal medullary granules. 

Release in Organs 

Very little is known concerning the mechanism of 
release of the cholinergic transmitter in the autonomic 
neuromuscular junctions. By studying the release of 
acetylcholine from the spontaneously beating or elec- 
trically driven rabbit's heart, it has been possible to 
show that the release is significantly higher at a 
faster heart rate. Thus a spontaneously beating heart 
with a mean rate of 56 per min. released 0.26 ± 
0.08 ^g per heart in 40 min. while electrically driven 
hearts with a mean rate of 210 per min. released 
0.97 ± 0.36 Kg per heart in 40 min. (32). 

The relea.se of acetylcholine from an organ does 
not necessarily mean that this substance originates 
from nervous tissue since it is known that even nerve- 
free tissue is able to synthesize and release acetylcho- 
line (17). '^" 

Most of the knowledge on the action of acet\ Icho- 
line and its release refers to the motor endplate which 
has been studied in detail from a chemical point of 
view as well as by electrophysiological techniques. 
There is hardly any douijt, however, that the mecha- 
nism of relea.se of the cholinergic transmitter from the 
postganglionic cholinergic nerves is similar in kind to 
that already outlined for the adrenergic transmitter. 
We may thus assume that the active transmitter is 
released at a terminal portion of the ner\e and acts 
directly in a chemical manner on the smooth muscle 
fibers. There is no reason to believe that the sijiooth 
muscle cells are directly innervated by cholinergic 
postganglionic fibers any more than they are by 
adrenergic fibers. 



AUTONOMIC NEUROEFFECTOR TRANSMISSION 



•^33 



RELEASE FROM ISOLATED NERVES. Although no experi- 
ments seem to have been made with stimulation of 
postgansrlionic cholinergic nerves, several authors 
have reported that stimulation of cholinergic pre- 
ganglionic nerves causes a release of acetylcholine 
(2, 10, 23, 79). It may be assumed that similar events 
take place during stimulation of postganglionic 
cholinergic nerves. 

The inhibitory action of atropine on the effect of 
cholinergic nerve stimulation has been shown to 
depend on blocking of the target cell to the released 
transmitter. It was demonstrated by Feldberg & 
Krayer (44) that atropine d oes ri otjnte rfere w ith the 
release as such. 

The failure of atropine to block the effect of stimu- 
lating the vagus nerve on the intestine may be due to 
an action of a transmitter different from acetylcholine, 
released from the enteric nerve system. The nature of 
this postulated transmitter is not known, but it should 
be recalled that substance P (51} occurs in the in- 
testine and is insensitive to atropine. 

Rosenblueth (113) has advanced the idea that the 
cholinergic nerve transmission proceeds in two stages 
of which the first is a release of acetylcholine followed 
ijy a .second in which the nerve transmitter subse- 
ciucntly forms ' paras)mpathin' which then acts 
directly on the target cell. 

Stimulation experiments on postganglionic cho- 
linergic nerves (short ciliary nerves) have shown that 
the optimum frequency is about 25 per sec. (89). As 
in the case of adrenergic nerves, prolonged stimula- 
tion caused Qnlj' slight signs of exhaustibility. Thus 
stimulation for i to 2 hours caused a sustained con- 
traction of the iris; thereafter the effect gradually 
declined. 

Removal of Trorumitter 

As early as 191 4 Dale (25) had assumed that 
acetylcholine was destroyed rapidly in the organism 
by some hydrolyzing enzyme. Such an enzyme was 
actually discovered by Loewi & Navratil (87) in ex- 
tracts of frog's heart. They also found that this .en^ 
zyme could i)c inhibited by physostiginine. This was 
in agreement with the results of earlier experiinents 
of Dixon & Brodie (35) and others who found that 
this drug increased some of the effects of parasympa- 
thetic nerve stimulation. Moreover, Loewi & Navratil 
were able to show that it increased the effect of the 
substance liberated from the frog's heart upon stimu- 
lation. The ' Vagusstoff ' thus behaved like a choline 
ester since it was a) inhibited by atropine which is a 



specific inhibitor, at least in small doses, and A) pro- 
tected by physostigmine which is known to inhibit 
choline esterase. It is generally assumed that the 
cholinergic transmitter is being inactivated locally to 
a great extent. Information about the distribution of 
cholinesterase in the peripheral tissue is accumulating 
rapidly as a result of the development of suitable 
methods. This includes important findings about the 
distribution of cholinesterase at the motor endplates 
and in the central nervous system (43, 77). It may be 
assumed that part of the transmitter released in 
peripheral organs, such as the smooth inuscle organs 
and glands, is diffusing out in the blood stream where 
it is rapidly inactivated by the cholinesterase present. 
It is also possible that cholinesterase is present in the 
target cells in amounts large enough to destroy any 
ainount of the transmitter diffusing into the cell. 



MECHANISM OF ACTION OF NEUROTRANSMITTERS 

The neurotransmitters exert direct action on target 
cells independently of whether or not the cells are 
autonomically innervated. This is shown by the pro- 
nounced action of the transmitter substances on nerve- 
free organs, like the placenta, or on denervated struc- 
tures. 

The mode of action of the neurotransmitters on the 
target cells has been much discussed. Clark (24) 
related the minimal effective doses of acetylcholine 
and epinephrine on the frog's heart and the frog's 
stomach to the total surface of the cells affected and 
arrived at the conclusion that while the effective dose 
of 0.02 /tig per gm covered a surface of al)out i cm- the 
total area of the cells was 6000 to 20000 cm-. For this 
reason it was obvious that the aQjive suijstance only 
needed to attack a minute part of the cell in order to 
elicit itijction. 

It is generally a.ssumed that the active substance, 
be it a neurotransmitter or a pharmacologically 
active drug of a different kind, has to unite in some 
way with the target cell before e.xerting its action. 
Often the sites of binding between the cell and the 
active molecule are referred to as receptors. According 
to Clark these postulated receptors, in or on the cell, 
occupy only a very small portion of the cell volume 
or surface. Morphological evidence for specific re- 
ceptor patches on the cell surface is still lacking, 
however. 

A discussion of the number of inolecules of a trans- 
mitter required to activate a single cell depends obvi- 
ously on the type of administration and on the sensi- 



234 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



tivitv of the cell. If 0.02 fig acetylcholine is necessary 
to inhibit one gram of frog's heart, as in Clark's ex- 
ample, the minimal effective amount per gm of tissue 
is about 10" molecules per cell. In isolated organs 
entirely different results may be obtained. Thus 
0.05 mjug epinephrine per ml suspension fluid is some- 
times enough to elicit an inhibitory effect on the 
fowl's isolated rectal cecum. In this case obviously a 
much smaller numijcr of molecules are capable of 
producing the action, since most of them are in the 
suspension fluid without contact with the organ. If it 
is assumed that i o per cent of the molecules are acting 
on I gm of organ containing 10'" cells, then the num- 
ber of molecules per cell will be only 10, provided 
that the active substance is distributed on all cells. 
This is probably not the case. All calculations of this 
kind therefore appear very dubious. 

It is conceivable that the neurotransmitter takes 
part in a chemical reaction sequence which is influ- 
enced thereby in a quantitative or even qualitative 
manner. Whether this action is initiated at specific 
receptor patches at the surface or at specific metabolic 
structure elements in the interior of the cell is not 
known. It may be recalled that there is good evidence 
for the permeation of neurotransmitters through cell 
membranes, since this is the basis for most of our 
information regarding their release. 

Elaborate schemes of receptor mechanisms have 
been presented by several authors and terms sug- 
gested for the postulated receptors. Since these efforts 
primarilv represent an attempt to put the known facts 
in a formal system but hardly contribute to our actual 
knowledge, these systems will not be dealt with here. 
Recent contributions to the discussion have been 
given by Zupancic (137) and Stephenson (iig). 

How the neurotransmitter elicits a relaxation or a 
contraction of the target cell is still ob.scure. It can be 
assumed that the active substance initiates or rein- 
forces soine process which eventually causes physico- 
chemical changes in the contractile material con- 
ducive to such effects. 

Attempts have been made to correlate the inhibi- 
tory actions of epinephrine with the formation of 
lactic acid (99), which is believed to be the metabolic 
product directly responsible for the inhiljitory action. 
The hypothesis obviously requires that the widely 
varying activity ratios of epinephrine and norepi- 
nephrine for an organ like the fowl's rectal cecum 
(from 4 to 200) are associated with corresponding 
variations in the formation of lactic acid in the react- 
ing target cells, a demonstration which has not been 
made 



On account of the large differences in action be- 
tween the levo- and dextroisomers of epinephrine, 
for instance, it has been inferred that the active sub- 
stance combines with an optically active constituent 
of the cell (122). Recent careful studies on the bio- 
logical activity of optical isomers of sympathomimetic 
amines have shown that the difference in action be- 
tween the isomers is even greater than has been 
hitherto recognized (90). 

These results suggest that the neurotransmitter is 
involved in enzymatic reactions, a conclusion which 
also appears most likely for other reasons. 

Another approach to the study of the mode of 
action of neurotransmitters on the target cell is based 
on the quantitative relationships between do.se and 
action. Such quantitation of the effects has ijeen used 
for the elaboration of formulae of \arious kinds. It is 
outside the scope of this article to discuss these studies. 
It may be said generally, however, that by applying 
this principle to single cells more information may be 
gained. In most cases the relationship between dose 
and action is approximately expressed by a rectangu- 
lar hyperbola. Its precise biological significance is 
not as yet clear. 

Summarizing, it may be concluded that not much 
more knowledge aijout the mode of action of the 
neurotransmitters on target cells has been gained 
since Langley's time when he ascribed the differenti- 
ating effect of the transmitter, relaxation or contrac- 
tion, to a receptor substance in the cell. 

A relevant question is whether two neurotrans- 
mitters released in the same organ act on the same or 
different cells and to what extent they interfere with 
one another's actions. Morison & Acheson (20) found 
similar hyperijolic concentration-action curves for 
epinephrine and acetylcholine on the nictitating mem- 
brane of the cat. When the two substances were in- 
jected together, their actions added up along the same 
curve. These results would seem to allow the impor- 
tant conclusion that the two neurotransmitters act 
independently by exerting separate actions. Whether 
or not these are on the same or different cells cannot 
be decided from these experiments. 



NEUROTRANSMITTERS IN BLOOD AND URINE 

It has been discussed above that some of the neuro- 
transmitter released at the autonomic postganglionic 
nerve endings passes bevond the target cells and 
reaches the l)lood stream. If this occurs to any con- 
siderable extent it should be possible to demonstrate 



AUTONOMIC NEUROEFFECTOR TRANSMISSION 



235 



the neurotransmitters in the blood. Such attempts 
have been made and there is some evidence for the 
opinion that the neurotransmitter of the adrenergic 
system normally occurs in small quantities in the 
blood. However, since the methods of demonstrating 
norepinephrine in blood require fairly large quanti- 
ties of blood and are rather laijorious (70, 91), they 
have not been widely used. Some indirect informa- 
tion has been obtained by studying the excretion in 
urine (67, 130). Even if norepinephrine occurs in 
peripheral blood it remains to be shown that it is 
derived from postganglionic nerves and not from the 



suprarenal medulla or from chromafhn cells. Proof of 
its overflow and passage into the blood has been given, 
however, by studies on the excretion in urine in 
adrenalectomized patients. In these patients the only 
important sources of norepinephrine can be the post- 
ganglionic nerves. For the same rea.sons as outlined 
above the excretion of acetylcholine, after treatment 
of the organism with physostigmine, will not allow 
any conclusions as to the release at the postganglionic 
nerve endings since acetylcholine is also released at 
manv other sites. 



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